Composition for holographic recording medium, and holographic recording medium

ABSTRACT

A holographic recording medium composition comprising component (e): a compound having an isocyanate group or an isocyanate-reactive functional group and further having a nitroxyl radical group, wherein component (e) contains component (e-1) below: 
     component (e-1): a compound having a heterobicyclic ring structure or a heterotricyclic ring structure, the heterobicyclic ring structure or the heterotricyclic ring structure being obtained by replacing a carbon atom in a bicyclic ring structure or a tricyclic ring structure by the nitroxyl radical group.

TECHNICAL FIELD

The present invention related to a composition for a holographicrecording medium, to a cured product for a holographic recording mediumthat is obtained by curing the holographic recording medium composition,and to a holographic recording medium using the holographic recordingmedium composition.

BACKGROUND ART

To achieve a further increase in the capacity and density of opticalrecording mediums, holographic optical recording mediums have beendeveloped, in which information is recorded as a hologram by changingthe refractive index of a recording layer according to the distributionof light intensity generated by light interference.

It has recently been contemplated that holographic recording mediumsdeveloped for memory applications are applied to optical elementapplications such as light guide plates for AR glasses (e.g., PTL 1).

The higher the performance indicator M/# of a holographic recordingmedium, the better for the following reasons.

In the memory applications, as the M/# increases, the recording capacityincreases.

In the optical element applications such as light guide plates for ARglasses, a higher M/# can give a larger viewing angle, less colorunevenness, and improved brightness.

PTL 2 discloses, as a technique for improving M/#, a method using acomposition containing a compound (e.g., TEMPOL) having both afunctional group chemically bonded to a matrix and a stable nitroxylradical group.

It is known that ABNO and AZADOs used in the present invention have highcatalytic activity for an oxidation reaction of alcohol.

PTL 1: U.S. Patent Application Publication No. 2017/0059759

PTL 2: U.S. Pat. No. 8,658,332

PTL 3: Japanese Unexamined Patent Application Publication No.2011-153076

PTL 4: International Publication No. WO2013/125688

NPL 1: KOBUNSHI RONBUNSHU (Japanese Journal of Polymer Science andTechnology), 2007, Vol. 64, No. 6, 329-342p (Public InterestIncorporated Association, The Society of Polymer Science, Japan)

Studies by the present inventors have revealed that the M/# of a mediumcontaining TEMPOL added thereto decreases under accelerated testconditions, i.e., the thermal stability of the medium is low. This maybe because a dormant species composed of a nitroxyl radical and apolymer radical in the medium is thermally cleaved and the polymerdiffuses in the matrix.

According to NPL 1, the dissociation energy of a NO—C bond in a dormantspecies derived from TEMPO is experimentally estimated to be 116 to 146kJ/mol. This means that the thermal cleavage proceeds under the heatingcondition of 70° C. or higher.

In the optical element applications such as light guide plates for ARglasses, a reduction in M/# is not preferred because it causes areduction in viewing angle, an increase in color unevenness, and areduction in brightness.

It is known that ABNO and AZADOs disclosed in PTL 3 and PTL 4 have highcatalytic activity for an oxidation reaction of alcohol. However, theuse of these compounds for holographic recording medium applications hasnot been reported.

SUMMARY OF INVENTION

It is an object of the present invention to provide a holographicrecording medium composition that contains a nitroxyl radicalgroup-containing additive added thereto in order to improve M/# andcapable of providing a higher M/# improving effect than TEMPOL and canprovide a holographic recording medium excellent in thermal stability ofM/#.

The present inventor has found that, in a holographic recording mediumto which ABNO or AZADO having a functional group chemically bonded tothe matrix is added, the M/# is higher than that when TEMPOL is addedand a reduction in M/# under accelerated test conditions is prevented.

The details of the above operational advantages are not clear but may beas follows.

In ABNO and AZADOs, steric hindrance around the nitroxyl group issmaller than that in TEMPOL. Therefore, the radical trapping efficiencyof the ABNO and AZADOs in the matrix of the holographic recording mediumis higher than that of the TEMPOL, so that the M/# increases.

In the case of TEMPOL, spatially twisted radicals are bonded. However,in the case of ABNO and AZADOs, it is unnecessary that radicals bondedbe twisted. Therefore, the energy of the dormant species is low, and theactivation energy of thermal cleavage is high, so that thermal cleavageis less likely to occur.

The present invention is summarized as follows.

[1] A holographic recording medium composition comprising component (e):a compound having an isocyanate group or an isocyanate-reactivefunctional group and further having a nitroxyl radical group, whereincomponent (e) contains component (e-1) below:

component (e-1): a compound having a heterobicyclic ring structure or aheterotricyclic ring structure, the heterobicyclic ring structure or theheterotricyclic ring structure being obtained by replacing a carbon atomin a bicyclic ring structure or a tricyclic ring structure by thenitroxyl radical group.

[2] A holographic recording medium composition comprising component (e):a compound having an isocyanate group or an isocyanate-reactivefunctional group and further having a nitroxyl radical group, whereincomponent (e) contains component (e-2) below:

component (e-2): a compound represented by formula (1) below:

wherein, in formula (I), C₁ and C₂ each represent a carbon atom; twoR^(X)s in formula (I) are each the same as R^(A) or are bonded togetherand represent a direct bond or a linking group, the direct bond or thelinking group covalently bridging the two carbon atoms C₁ and C₂; and,when R^(X) and R^(X) represent the linking group, the linking group isrepresented by —C(—R^(A))₂— or —C(—R^(A))₂—C(—R^(A))₂—,

wherein, in formula (I) and the linking group, each R^(A) represents asubstituent selected from a hydrogen atom, halogen atoms, C1-12 alkylgroups, a hydroxy group, hydroxy(C1-12 alkyl) groups, an amino group,amino(C1-12 alkyl) groups, an isocyanate group, and isocyanate(C1-12alkyl) groups; in formula (I) and the linking group, one or two R^(A)sare bonded to each carbon atom in a bicyclic ring structure or atricyclic ring structure; when the number of substituents on a carbonatom is two or more, the substituents may be the same or different; and,in formula (I) and the linking group, the plurality of R^(A)s may be thesame or different,

provided that at least one of R^(A)s in formula (I) and the linkinggroup is at least one substituent selected from a hydroxy group,hydroxy(C1-12 alkyl) groups, an amino group, amino(C1-12 alkyl) groups,and an isocyanate group.

[3] The holographic recording medium composition according to [1] or[2], wherein the holographic recording medium composition furthercomprises: a matrix resin; component (c): a polymerizable monomer; andcomponent (d): a photopolymerization initiator.

[4] The holographic recording medium composition according to [3],wherein the matrix resin is obtained through the reaction of component(a): an isocyanate group-containing compound and component (b): anisocyanate-reactive functional group-containing compound.[5] A holographic recording medium composition comprising components (a)to (e) below, wherein component (e) contains component (e-1) below:

component (a): an isocyanate group-containing compound;

component (b): an isocyanate-reactive functional group-containingcompound;

component (c): a polymerizable monomer;

component (d): a photopolymerization initiator;

component (e): a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group; and

component (e-1): a compound having a heterobicyclic ring structure or aheterotricyclic ring structure, the heterobicyclic ring structure or theheterotricyclic ring structure being obtained by replacing a carbon atomin a bicyclic ring structure or a tricyclic ring structure by thenitroxyl radical group.

[6] A holographic recording medium composition comprising components (a)to (e) below, wherein component (e) contains component (e-2) below:

component (a): an isocyanate group-containing compound;

component (b): an isocyanate-reactive functional group-containingcompound;

component (c): a polymerizable monomer;

component (d): a photopolymerization initiator;

component (e): a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group; and

component (e-2): a compound represented by formula (I) below:

wherein, in formula (I), C₁ and C₂ each represent a carbon atom; twoR^(X)s in formula (I) are each the same as R^(A) or are bonded togetherand represent a direct bond or a linking group, the direct bond or thelinking group covalently bridging the two carbon atoms C₁ and C₂; and,when R^(X) and R^(X) represent the linking group, the linking group isrepresented by —C(—R^(A))₂— or —C(—R^(A))₂—C(—R^(A))₂—,

wherein, in formula (I) and the linking group, each R^(A) represents asubstituent selected from a hydrogen atom, halogen atoms, C1-12 alkylgroups, a hydroxy group, hydroxy(C1-12 alkyl) groups, an amino group,amino(C1-12 alkyl) groups, an isocyanate group, and isocyanate(C1-12alkyl) groups; in formula (I) and the linking group, one or two R^(A)sare bonded to each carbon atom in a bicyclic ring structure or atricyclic ring structure; when the number of substituents on a carbonatom is two or more, the substituents may be the same or different; and,in formula (I) and the linking group, the plurality of R^(a)s may be thesame or different,

provided that at least one of R^(A)s in formula (I) and the linkinggroup is at least one substituent selected from a hydroxy group,hydroxy(C1-12 alkyl) groups, an amino group, amino(C1-12 alkyl) groups,and an isocyanate group.

[7] The holographic recording medium composition according to [2] or[6], wherein the compound represented by formula (I) is a compoundrepresented by one of formulas (Ia) to (Ic) below:

wherein, in formulas (Ia) to (Ic), each R^(A) is the same as that informula (I).

[8] A holographic recording medium composition comprising components (a)to (e) below, wherein component (e) contains component (e-3) below:

component (a): an isocyanate group-containing compound;

component (b): an isocyanate-reactive functional group-containingcompound;

component (c): a polymerizable monomer;

component (d): a photopolymerization initiator;

component (e): a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group;

component (e-3): a compound in which the nitroxyl radical group is notsterically hindered and which is selected from (e-3-1) to (e-3-3) below:

(e-3-1): a compound in which the number of alkyl groups covalentlybonded to respective two atoms covalently bonded to the nitrogen atom ofthe nitroxyl radical group is 0 or 1;

(e-3-2): a compound in which at least one of the two carbon atomscovalently bonded to the nitrogen atom of the nitroxyl radical group iscovalently bonded to at least one hydrogen or halogen atom; and

(e-3-3): a compound in which both of the two carbon atoms covalentlybonded to the nitrogen atom of the nitroxyl radical group are eachcovalently bonded to at least one hydrogen or halogen atom.

[9] A holographic recording medium composition comprising components (a)to (e) below, wherein component (e) contains component (e-4) below:

component (a): an isocyanate group-containing compound;

component (b): an isocyanate-reactive functional group-containingcompound;

component (c): a polymerizable monomer;

component (d): a photopolymerization initiator;

component (e): a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group; and

component (e-4): a compound including a nitroxyl radicalgroup-containing mother compound substituted with the isocyanate groupor the isocyanate-reactive functional group, the nitroxyl radicalgroup-containing mother compound having a redox potential of 280 mV orless.

[10] The holographic recording medium composition according to any oneof [4] to [9], wherein the ratio of the total weight of a propyleneglycol unit and a tetramethylene glycol unit included in component (b)to the total weight of component (a) and component (b) is 30% or less.[11] The holographic recording medium composition according to [10],wherein the ratio of the weight of a caprolactone unit contained incomponent (b) to the total weight of component (a) and component (b) is20% or more.[12] The holographic recording medium composition according to any oneof [4] to [11], further comprising component (f) below:

component (f): a curing catalyst.

[13] The holographic recording medium composition according to any oneof [3] to [12], wherein the molar ratio of component (e) to component(d) (component (e)/component (d)) in the composition is 0.1 or more and10 or less.

[14] The holographic recording medium composition according to any oneof [3] to [13], wherein the content of component (c) in the compositionis 0.1% by weight or more and 80% by weight or less, and wherein theratio of the amount of component (d) to the amount of component (c) is0.1% by weight or more and 20% by weight or less.[15] The holographic recording medium composition according to any oneof [4] to [14], wherein the total content of component (a) and component(b) in the composition is 0.1% by weight or more and 99.9% by weight orless, and wherein the ratio of the number of isocyanate-reactivefunctional groups contained in component (b) to the number of isocyanategroups contained in component (a) is 0.1 or more and 10.0 or less.[16] A cured product for a holographic recording medium, wherein thecured product is obtained by curing the holographic recording mediumcomposition according to any one of [1] to [15].[17] A stacked body for a holographic recording medium, the stacked bodycomprising: the cured product for a holographic recording mediumaccording to [16], the cured product being used as a recording layer; asupport disposed on the upper and/or lower sides of the recording layer.[18] A holographic recording medium obtained by subjecting the curedproduct for a holographic recording medium according to [16] or thestacked body for a holographic recording medium according to [17] tointerference exposure.[19] The holographic recording medium according to [18], wherein theholographic recording medium is a light guide plate for AR glasses.

Advantageous Effects of Invention

The present invention can provide a holographic recording medium whichcan have a higher M/# than a conventional holographic recording mediumand is excellent in thermal stability of M/# and in which a reduction inM/# with time in a high-temperature environment is prevented.

Therefore, particularly in the AR glass light guide plate applications,an increase in viewing angle, a reduction in color unevenness, animprovement in brightness, and an increase in thermal stability can beachieved.

In the memory applications also, an increase in recording capacity andan increase in thermal stability of signals can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a structural diagram showing the outline of a device used forholographic recording in Examples.

FIG. 2 is a graph showing the relation between M/# and the amount of anadditive added (additive/component (d) (molar ratio)) in Examples 1 to10 and Comparative Examples 2 to 9.

FIGS. 3A, 3B, and 3C show the results of evaluation of the archival lifeof a holographic recording medium in Comparative Example 4. FIG. 3 a isa graph showing a change in M/# over time. FIG. 3 b is a graph showingthe diffraction efficiency of sample No. 1 at the beginning of the test.FIG. 3 c is a graph showing the diffraction efficiency of sample No. 1after the 800 hour accelerated test.

FIGS. 4A, 4B, and 4C show the results of evaluation of the archival lifeof a holographic recording medium in Comparative Example 7. FIG. 4 a isa graph showing a change in M/# over time. FIG. 4 b is a graph showingthe diffraction efficiency of sample No. 1 at the beginning of the test.FIG. 4 c is a graph showing the diffraction efficiency of sample No. 1after the 870 hour accelerated test.

FIGS. 5A, 5B, and 5C show the results of evaluation of the archival lifeof a holographic recording medium in Example 3. FIG. 5 a is a graphshowing a change in M/# over time. FIG. 5 b is a graph showing thediffraction efficiency of sample No. 1 at the beginning of the test.FIG. 5 c is a graph showing the diffraction efficiency of sample No. 1after the 800 hour accelerated test.

FIGS. 6A, 6B, and 6C show the results of evaluation of the archival lifeof a holographic recording medium in Example 7. FIG. 6 a is a graphshowing a change in M/# over time. FIG. 6 b is a graph showing thediffraction efficiency of sample No. 1 at the beginning of the test.FIG. 6 c is a graph showing the diffraction efficiency of sample No. 1after the 857 hour accelerated test.

FIGS. 7A, 7B, and 7C show the results of evaluation of the archival lifeof a holographic recording medium in Example 10. FIG. 7 a is a graphshowing a change in M/# over time. FIG. 7 b is a graph showing thediffraction efficiency of sample No. 1 at the beginning of the test.FIG. 7 c is a graph showing the diffraction efficiency of sample No. 1after the 800 hour accelerated test.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will be described in detail. Thefollowing articles, methods, etc. are examples (representative examples)of the embodiments of the present invention, and the present inventionis not limited to the features of the embodiments so long as theinvention does not depart from the scope thereof.

[Holographic Recording Medium Composition]

A holographic recording medium composition according to one embodimentof the present invention is a holographic recording medium compositioncontaining component (e): a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group, and component (e) contains component (e-1) below.

component (e-1): a compound having a heterobicyclic ring structure or aheterotricyclic ring structure, the heterobicyclic ring structure or theheterotricyclic ring structure being obtained by replacing a carbon atomin a bicyclic ring structure or a tricyclic ring structure by thenitroxyl radical group.

A holographic recording medium composition according to anotherembodiment of the present invention is a holographic recording mediumcomposition containing component (e): a compound having an isocyanategroup or an isocyanate-reactive functional group and further having anitroxyl radical group, and component (e) contains component (e-2)below.

wherein, in formula (I), C₁ and C₂ each represent a carbon atom; twoR^(X)s in formula (I) are each the same as R^(A) or are bonded togetherand represent a direct bond or a linking group, the direct bond or thelinking group covalently bridging the two carbon atoms C₁ and C₂; and,when R^(X) and R^(X) represent the linking group, the linking group isrepresented by —C(—R^(A))₂— or —C(—R^(A))₂—C(—R^(A))₂—,

wherein, in formula (I) and the linking group, each R^(A) represents asubstituent selected from a hydrogen atom, halogen atoms, C1-12 alkylgroups, a hydroxy group, hydroxy(C1-12 alkyl) groups, an amino group,amino(C1-12 alkyl) groups, an isocyanate group, and isocyanate(C1-12alkyl) groups; in formula (I) and the linking group, one or two R^(A)sare bonded to each carbon atom in a bicyclic ring structure or atricyclic ring structure; when the number of substituents on a carbonatom is two or more, the substituents may be the same or different; and,in formula (I) and the linking group, the plurality of R^(A)s may be thesame or different,

provided that at least one of R^(A)s in formula (I) and the linkinggroup is at least one substituent selected from a hydroxy group,hydroxy(C1-12 alkyl) groups, an amino group, amino(C1-12 alkyl) groups,and an isocyanate group.

Each of the above holographic recording medium compositions may furthercontain a matrix resin, component (c): a polymerizable monomer, andcomponent (d): a photopolymerization initiator.

In this case, the matrix resin may be obtained through the reaction ofcomponent (a): an isocyanate group-containing compound and component(b): an isocyanate-reactive functional group-containing compound.

A holographic recording medium composition according to anotherembodiment of the present invention is a holographic recording mediumcomposition containing components (a) to (e) below, and component (e)contains component (e-1) below.

component (a): an isocyanate group-containing compound;

component (b): an isocyanate-reactive functional group-containingcompound;

component (c): a polymerizable monomer;

component (d): a photopolymerization initiator;

component (e): a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group; and

component (e-1): a compound having a heterobicyclic ring structure or aheterotricyclic ring structure, the heterobicyclic ring structure or theheterotricyclic ring structure being obtained by replacing a carbon atomin a bicyclic ring structure or a tricyclic ring structure by thenitroxyl radical group.

A holographic recording medium composition according to anotherembodiment of the present invention is a holographic recording mediumcomposition containing components (a) to (e) below, and component (e)contains component (e-2) below.

component (a): an isocyanate group-containing compound;

component (b): an isocyanate-reactive functional group-containingcompound;

component (c): a polymerizable monomer;

component (d): a photopolymerization initiator;

component (e): a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group; and

component (e-2): a compound represented by formula (I) below:

wherein, in formula (I), C₁ and C₂ each represent a carbon atom; twoR^(X)s in formula (I) are each the same as R^(A) or are bonded togetherand represent a direct bond or a linking group, the direct bond or thelinking group covalently bridging the two carbon atoms C₁ and C₂; and,when R^(X) and R^(X) represent the linking group, the linking group isrepresented by —C(—R^(A))₂— or —C(—R^(A))₂—C(—R^(A))₂—,

wherein, in formula (I) and the linking group, each R^(A) represents asubstituent selected from a hydrogen atom, halogen atoms, C1-12 alkylgroups, a hydroxy group, hydroxy(C1-12 alkyl) groups, an amino group,amino(C1-12 alkyl) groups, an isocyanate group, and isocyanate(C1-12alkyl) groups; in formula (I) and the linking group, one or two R^(A)sare bonded to each carbon atom in a bicyclic ring structure or atricyclic ring structure; when the number of substituents on a carbonatom is two or more, the substituents may be the same or different; and,in formula (I) and the linking group, the plurality of R^(a)s may be thesame or different,

provided that at least one of R^(A)s in formula (I) and the linkinggroup is at least one substituent selected from a hydroxy group,hydroxy(C1-12 alkyl) groups, an amino group, amino(C1-12 alkyl) groups,and an isocyanate group.

A holographic recording medium composition according to anotherembodiment of the present invention is a holographic recording mediumcomposition containing components (a) to (e) below, and component (e)contains component (e-3) below.

component (a): an isocyanate group-containing compound;

component (b): an isocyanate-reactive functional group-containingcompound;

component (c): a polymerizable monomer;

component (d): a photopolymerization initiator;

component (e): a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group;

component (e-3): a compound in which the nitroxyl radical group is notsterically hindered and which is selected from (e-3-1) to (e-3-3) below:

(e-3-1): a compound in which the number of alkyl groups covalentlybonded to respective two atoms covalently bonded to the nitrogen atom ofthe nitroxyl radical group is 0 or 1;

(e-3-2): a compound in which at least one of the two carbon atomscovalently bonded to the nitrogen atom of the nitroxyl radical group iscovalently bonded to at least one hydrogen or halogen atom; and

(e-3-3): a compound in which both of the two carbon atoms covalentlybonded to the nitrogen atom of the nitroxyl radical group are eachcovalently bonded to at least one hydrogen or halogen atom.

A holographic recording medium composition according to anotherembodiment of the present invention is a holographic recording mediumcomposition containing components (a) to (e) below, and component (e)contains component (e-4) below.

component (a): an isocyanate group-containing compound;

component (b): an isocyanate-reactive functional group-containingcompound;

component (c): a polymerizable monomer;

component (d): a photopolymerization initiator;

component (e): a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group; and

component (e-4): a compound including a nitroxyl radicalgroup-containing mother compound substituted with the isocyanate groupor the isocyanate-reactive functional group, the nitroxyl radicalgroup-containing mother compound having a redox potential of 280 mV orless.

<Component (a)>

The isocyanate group-containing compound denoted as Component (a) ispreferably a component that reacts with the isocyanate-reactivefunctional group-containing compound (component (b)) in the presence ofa curing catalyst (component (f)) described later to form a resinmatrix.

An isocyanate group ratio in the molecule of the isocyanategroup-containing compound is preferably 50% by weight or less, morepreferably 47% by weight or less, and still more preferably 45% byweight or less. The lower limit of the isocyanate group ratio isgenerally 0.1% by weight and preferably 1% by weight. When theisocyanate group ratio is less than the above upper limit, a holographicrecording medium to be formed is unlikely to become turbid, and opticaluniformity is obtained. When the isocyanate group ratio is more than theabove lower limit, the hardness and glass transition temperature of theresin matrix increase, and loss of records can be prevented.

In the present invention, the isocyanate group ratio indicates the ratioof the isocyanate group to the whole amount of the isocyanategroup-containing compound used. The isocyanate group ratio in theisocyanate group-containing compound is determined from the followingformula. The molecular weight of the isocyanate group is 42.(42×the number of isocyanate groups/the molecular weight of theisocyanate group-containing compound)×100

No particular limitation is imposed on the type of isocyanategroup-containing compound, and the compound may have an aromaticskeleton, an araliphatic skeleton, an aliphatic skeleton, or analicyclic skeleton.

The isocyanate group-containing compound may have in its molecule oneisocyanate group or two or more isocyanate groups. The isocyanategroup-containing compound has preferably two or more isocyanate groups.A recording layer having good record retainability can be obtained usinga three-dimensional crosslinked matrix obtained from a compound havingtwo or more isocyanate groups in its molecule (component (a)) and acompound having three or more isocyanate-reactive functional groups inits molecule (component (b)) or a three-dimensional crosslinked matrixobtained from a compound having three or more isocyanate groups in itsmolecule (component (a)) and a compound having two or moreisocyanate-reactive functional groups in its molecule (component (b)).

Examples of the isocyanate group-containing compound include isocyanicacid, butyl isocyanate, octyl isocyanate, butyl diisocyanate,hexamethylene diisocyanate (HMDI), isophorone diisocyanate (IPDI),1,8-diisocyanato-4-(isocyanatomethyl)octane, 2,2,4- and/or2,4,4-trimethylhexamethylene diisocyanate, isomericbis(4,4′-isocyanatocyclohexyl)methanes and mixtures thereof having anydesired isomer content, isocyanatomethyl-1,8-octanediisocyanate,1,4-cyclohexylene diisocyanate, isomeric cyclohexanedimethylenediisocyanates, 1,4-phenylene diisocyanate, 2,4- and/or 2,6-toluenediisocyanate, 1,5-naphthylene diisocyanate, 2,4′- and/or4,4′-diphenylmethane diisocyanate and/or triphenylmethane4,4′,4″-triisocyanate.

As the isocyanate group-containing compound, it is also possible to usean isocyanate derivative having a urethane structure, a urea structure,a carbodiimide structure, an acrylic urea structure, an isocyanuratestructure, an allophanate structure, a biuret structure, anoxadiazinetrione structure, a uretdione structure, and/or animinooxadiazinedione structure.

Any one of them may be used alone, or any combination of two or more ofthem may be used at any ratio.

Among the above compounds, isocyanate group-containing aliphaticcompounds and isocyanate group-containing alicyclic compounds arepreferred because they resist coloration.

<Component (b)>

The isocyanate-reactive functional group-containing compound denoted ascomponent (b) is a compound having active hydrogen (anisocyanate-reactive functional group) that is involved in a chainextension reaction with the isocyanate group-containing compound denotedas component (a).

Examples of the isocyanate-reactive functional group include a hydroxygroup, an amino group, and a mercapto group.

The isocyanate-reactive functional group-containing compound may containin its molecule one isocyanate-reactive functional group or two or moreisocyanate-reactive functional groups. The isocyanate-reactivefunctional group-containing compound preferably contains two or moreisocyanate-reactive functional groups. When two or moreisocyanate-reactive functional groups are contained, theisocyanate-reactive functional groups contained in one molecule may beof the same type or different types.

The number average molecular weight of the isocyanate-reactivefunctional group-containing compound is generally 50 or more, preferably100 or more, and more preferably 150 or more and is generally 50000 orless, 10000 or less, and more preferably 5000 or less. When the numberaverage molecular weight of the isocyanate-reactive functionalgroup-containing compound is equal to or more than the above lowerlimit, the crosslink density is low, and a reduction in recording ratecan be prevented. When the number average molecular weight of theisocyanate-reactive functional group-containing compound is equal to orless than the above upper limit, its compatibility with anothercomponent increases, and the crosslink density increases, so that lossof recorded contents can be prevented.

The number average molecular weight of component (b) is a value measuredby gel permeation chromatography (GPC).

(Hydroxy Group-Containing Compound)

A compound having a hydroxy group as the isocyanate-reactive functionalgroup may be used. The hydroxy group-containing compound may be anycompound having in its molecule at least one hydroxy group, and it ispreferable that the compound has two or more hydroxy groups. Examples ofsuch a compound include: glycols such as ethylene glycol, triethyleneglycol, diethylene glycol, polyethylene glycol, propylene glycol,polypropylene glycol (PPG), and neopentyl glycol; diols such asbutanediol, pentanediol, hexanediol, heptanediol, tetramethylene glycol(TMG), and polytetramethylene glycol (PTMG); bisphenols and compoundsobtained by modifying these polyfunctional alcohols with apolyethyleneoxy chain or a polypropyleneoxy chain; triols such asglycerin, trimethylolpropane, butanetriol, pentanetriol, hexanetriol,and decanetriol and compounds obtained by modifying these polyfunctionalalcohols with a polyethyleneoxy chain or a polypropyleneoxy chain;polyfunctional polyoxybutylenes; polyfunctional polycaprolactones;polyfunctional polyesters; polyfunctional polycarbonates; andpolyfunctional polypropylene glycols. Any one of them may be used alone,or any combination of two or more of them may be used at any ratio.

The number average molecular weight of the hydroxy group-containingcompound is generally 50 or more, preferably 100 or more, and morepreferably 150 or more and is generally 50000 or less, 10000 or less,and more preferably 5000 or less. When the number average molecularweight of the hydroxy group-containing compound is equal to or more thanthe above lower limit, the crosslink density is low, and a reduction inrecording rate can be prevented. When the number average molecularweight of the hydroxy group-containing compound is equal to or less thanthe above upper limit, its compatibility with another component isimproved, and the crosslink density increases, so that loss of recordedcontents can be prevented.

(Amino Group-Containing Compound)

A compound having an amino group as the isocyanate-reactive functionalgroup may be used. The amino group-containing compound may be anycompound having in its molecule at least one amino group, and it ispreferable that the compound has two or more amino groups. Examples ofsuch a compound include: aliphatic amines such as ethylenediamine,diethylenetriamine, triethylenetetramine, and hexamethylenediamine;alicyclic amines such as isophoronediamine, menthanediamine, and4,4′-diaminodicyclohexylmethane; and aromatic amines such asm-xylylenediamine, diaminodiphenylmethane, and m-phenylenediamine. Anyone of them may be used alone, or any combination of two or more of themmay be used at any ratio.

The number average molecular weight of the amino group-containingcompound is generally 50 or more, preferably 100 or more, and morepreferably 150 or more and is generally 50000 or less, preferably 10000or less, and more preferably 5000 or less. When the number averagemolecular weight of the amino group-containing compound is equal to ormore than the above lower limit, the crosslink density is low, and areduction in the recording rate can be prevented. When the numberaverage molecular weight of the amino group-containing compound is equalto or less than the above upper limit, its compatibility with anothercomponent is improved, and the crosslink density increases, so that lossof recorded contents can be prevented.

(Mercapto Group-Containing Compound)

A compound having a mercapto group as the isocyanate-reactive functionalgroup may be used. The mercapto group-containing compound may be anycompound having in its molecule at least one mercapto group, and it ispreferable that the compound has two or more mercapto groups. Examplesof such a compound include 1,3-butanedithiol, 1,4-butanedithiol,2,3-butanedithiol, 1,2-benzenedithiol, 1,3-benzenedithiol,1,4-benzenedithiol, 1,10-decanedithiol, 1,2-ethanedithiol,1,6-hexanedithiol, 1,9-nonanedithiol, pentaerythritoltetrakis(3-mercaptopropionate), pentaerythritoltetrakis(2-mercaptoacetate), pentaerythritoltetrakis(3-mercaptobutyrate), trimethylolpropanetris(3-mercaptopropionate), dipentaerythritolhexakis(3-mercaptopropionate), and 1,4-bis(3-mercaptobutyryloxy)butane.Any one of them may be used alone, or any combination of two or more ofthem may be used at any ratio.

The number average molecular weight of the mercapto group-containingcompound is generally 50 or more, preferably 100 or more, and morepreferably 150 or more and is generally 50000 or less, 10000 or less,and more preferably 5000 or less. When the number average molecularweight of the mercapto group-containing compound is equal to or morethan the above lower limit, the crosslink density is low, and areduction in the recording rate can be prevented. When the numberaverage molecular weight of the mercapto group-containing compound isequal to or less than the above upper limit, its compatibility withanother component is improved, and the crosslink density increases, sothat loss of recorded contents can be prevented.

(Component (b) Preferred in the Present Invention)

The present inventor has found that, when a compound having anisocyanate group or an isocyanate-reactive functional group and furtherhaving a nitroxyl radical group is used as component (e), theholographic recording medium is tinted because of a propylene glycol(PG) unit and a tetramethylene glycol (TMG) unit contained in component(b). From the viewpoint of reducing coloration of the holographicrecording medium, the smaller the ratio of the total weight of the PGunit and the TMG unit contained in component (b) (which may behereinafter denoted as “(PG+TMG)/((a)+(b))”), the further the colorationof the holographic recording medium can be reduced. From the viewpointof reducing coloration of the holographic recording medium, the ratio(PG+TMG)/((a)+(b)) in the holographic recording medium composition ofthe present invention is preferably 30% or less, particularly preferably27% or less, especially preferably 25% or less, and most preferably 0%(no PG unit and no TMG unit are contained).

In terms of reducing the ratio (PG+TMG)/(((a)+(b))) to the above upperlimit or less, it is not preferable to use, as component (b), a compoundhaving PG units or TMG units such as polypropylene glycol (PPG) orpolytetramethylene glycol (PTMG). Therefore, when a compound having PGunits or TMG units is used, the compound is used such that the ratio(PG+TMG)/((a)+(b)) in the composition is equal to or less than the aboveupper limit.

From the viewpoint of reducing coloration, it is preferable to usepolycaprolactone as component (b). When polycaprolactone is used ascomponent (b), it is preferable to design the chemical composition suchthat the ratio of the weight of a caprolactone (CL) unit contained incomponent (b) to the total weight of component (a) and component (b) inthe holographic recording medium composition of the present invention(hereinafter, the ratio may be denoted as “(CL)/((a)+(b))”) ispreferably 20% or more, particularly preferably 25% or more, andespecially preferably 30 to 70%. Here, the CL unit is a unit (open-chainunit) derived from caprolactone contained in the polycaprolactone.

Examples of the polycaprolactone used as component (b) includepolycaprolactonepolyols (such as polycaprolactonediol andpolycaprolactonetriol) obtained by subjecting ε-caprolactone toring-opening polymerization in the presence of a polyhydric alcohol suchas ethylene glycol, diethylene glycol, triethylene glycol, propyleneglycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol,hexamethylene glycol, methylpentanediol, 2,4-diethylpentanediol,neopentyl glycol, 2-ethyl-1,3-hexanediol, trimethylolpropane,ditrimethylolpropane, pentaerythritol, or dipentaerythritol or a diolsuch as polypropylene glycol or polytetramethylene glycol (PTMG). Anyone of them may be used alone, or any combination of two or more of themmay be used at any ratio.

Preferably, the holographic recording medium composition of the presentinvention contains any of the above-listed polycaprolactonepolyols ascomponent (b). For example, it is preferable that only apolycaprolactonepolyol is used as component (b) or a combination of apolycaprolactonepolyol and another component (b) is used. Particularlypreferably, only a polycaprolactonepolyol is used as component (b) toadjust the ratio (CL)/((a)+(b)) in the composition to the above lowerlimit or higher.

When the ratio (PG+TMG)/((a)+(b)) in the composition is equal to or lessthan the above upper limit, a polyol having a PG unit or a TMG unit suchas polypropylene glycol may be modified with, for example,ε-caprolactone to produce a compound having a CL unit introducedtherein, and this compound can be used as component (b).

<Component (c)>

The polymerizable monomer denoted as component (c) is a compound thatcan be polymerized using the photopolymerization initiator denoted ascomponent (d) described later. Component (c) is a monomer compound to bepolymerized during recording and/or postexposure. No particularlimitation is imposed on the type of polymerizable monomer used for theholographic recording medium composition of the present invention, andan appropriate compound can be selected from known compounds. Examplesof the polymerizable monomer include cationically polymerizablemonomers, anionically polymerizable monomers, and radicallypolymerizable monomers. Any of these monomers can be used, and two ormore of them may be used in combination.

Component (c) used is preferably a radically polymerizable monomerbecause the radically polymerizable monomer is unlikely to inhibit thereaction through which the isocyanate group-containing compound and theisocyanate-reactive functional group-containing compound form thematrix.

(Cationically Polymerizable Monomer)

Examples of the cationically polymerizable monomer include epoxycompounds, oxetane compounds, oxolane compounds, cyclic acetalcompounds, cyclic lactone compounds, thiirane compounds, thietanecompounds, vinyl ether compounds, spiro orthoester compounds,ethylenically unsaturated compounds, cyclic ether compounds, cyclicthioether compounds, and vinyl compounds. Any one of these cationicallypolymerizable monomers may be used alone, or any combination of two ormore of them may be used at any ratio.

(Anionically Polymerizable Monomer)

Examples of the anionically polymerizable monomer include hydrocarbonmonomers and polar monomers.

Examples of the hydrocarbon monomers include styrene, α-methylstyrene,butadiene, isoprene, vinylpyridine, vinylanthracene, and derivativesthereof.

Examples of the polar monomers including methacrylates, acrylates, vinylketones, isopropenyl ketones, and other polar monomers.

Any one of these anionically polymerizable monomers may be used alone,or any combination of two or more of them may be used at any ratio.

(Radically Polymerizable Monomer)

Examples of the radically polymerizable monomer include (meth)acryloylgroup-containing compounds, (meth)acrylamides, vinyl esters, vinylcompounds, styrenes, and spiro ring-containing compounds. Any of theseradically polymerizable monomers may be used alone, or any combinationof two or more of them may be used at any ratio.

In the present description, methacrylic and acrylic are collectivelyreferred to as (meth)acrylic.

Among the above compounds, (meth)acryloyl group-containing compounds arepreferred in terms of steric hindrance during radical polymerization.

(Molecular Weight of Polymerizable Monomer)

The molecular weight of the polymerizable monomer used for theholographic recording medium composition of the present invention isgenerally 80 or more, preferably 150 or more, and more preferably 300 ormore and is generally 3000 or less, preferably 2500 or less, and morepreferably 2000 or less. When the molecular weight is equal to or morethan the above lower limit, the rate of shrinkage due to polymerizationby irradiation with light during holographic data recording can bereduced. When the molecular weight is equal to or less than the aboveupper limit, the mobility of the polymerizable monomer in a recordinglayer using the holographic recording medium composition is high, andthe polymerizable monomer can easily diffuse, so that sufficientdiffraction efficiency can be obtained.

(Refractive Index of Polymerizable Monomer)

The refractive index of the polymerizable monomer at a wavelength oflight applied to the holographic recording medium (at, for example, arecording wavelength) is generally 1.50 or more, preferably 1.52 ormore, and still more preferably 1.55 or more and is generally 1.80 orless and preferably 1.78 or less. If the refractive index is excessivelysmall, the diffraction efficiency is not sufficient, and M/# may not besufficient. If the refractive index is excessively large, the differencein refractive index between the polymerizable monomer and the resinmatrix is excessively large. In this case, scattering is significant,and this causes a reduction in transmittance, so that the visibility islowered in the case of AR glasses application.

The refractive index evaluated at a shorter wavelength is larger. Asample whose refractive index is relatively high at short wavelengthsexhibits a relatively high refractive index also at long wavelengths,and this relation is not reversed. It is therefore possible to predictthe refractive index at the recording wavelength by evaluating therefractive index at a wavelength other than the recording wavelength.

When a sample of the polymerizable monomer is liquid, its refractiveindex can be measured by the minimum deviation method, the criticalangle method, the V-block method, etc.

When the sample is solid, the refractive index of the polymerizablemonomer can be determined by dissolving the compound in an appropriatesolvent to prepare a solution, measuring the refractive index of thesolution, and extrapolating the refractive index to a point where thecontent of the compound is 100% to thereby determine the refractiveindex.

The polymerizable monomer having a high refractive index is preferably acompound having a halogen atom (such as iodine, chlorine, or bromine) inits molecule or a compound having a heteroatom (such as nitrogen,sulfur, or oxygen). Of these, a compound having a heterocyclic structureis more preferable.

It is preferable, in order to ensure solubility in a solvent or thematrix, that the number of heteroatoms in the heterocyclic structureincluded in the molecule of each of these compounds is two or less. Whenthe number of heteroatoms in the molecule is two or less, the solubilityis improved, and a uniform recording layer can be obtained.

If the structural regularity of the heterocycle in the molecule is high,coloration or a reduction in solubility may occur due to stacking.Therefore, a heteroaryl group formed by condensation of two or morerings is more preferable.

(Compound i)

One preferred example of the polymerizable monomer is compound i that isa (meth)acrylic monomer described in Japanese Unexamined PatentApplication Publication No. 2010-018606 and represented by the following(formula a).

In (formula a), A is a ring optionally having a substituent. Ar is a(hetero)aryl group which optionally has a substituent and in which twoor more rings are condensed. R¹ is a hydrogen atom or a methyl group. pis an integer of 1 to 7. When p is 2 or more, the plurality of Ar's maybe the same or different. Suppose that A is an aromatic heterocycle andAr is a heteroaryl group which optionally has a substituent and in whichtwo or more rings are condensed. Then, in a structure including A and Arconnected to each other, partial structures of A and Ar that areconnected directly to each other include no heteroatom.

Examples of the (meth)acrylic monomer represented by (formula a) aboveinclude 2-(1-thianthrenyl)-1-phenyl (meth)acrylate,2-(2-benzothiophenyl)-1-phenyl (meth)acrylate,2-(4-dibenzofuranyl)-1-phenyl (meth)acrylate,2-(4-dibenzothiophenyl)-1-phenyl (meth) acrylate,3-(3-benzothiophenyl)-1-phenyl (meth)acrylate,3-(4-dibenzofuranyl)-1-phenyl (meth) acrylate,3-(4-dibenzothiophenyl)-1-phenyl (meth)acrylate,4-(1-thianthrenyl)-1-phenyl (meth)acrylate,4-(4-dibenzothiophenyl)-1-phenyl (meth)acrylate,2,4-bis(1-thianthrenyl)-1-phenyl (meth)acrylate,2,4-bis(2-dibenzothiophenyl)-1-phenyl (meth)acrylate,2,4-bis(3-benzothiophenyl)-1-phenyl (meth)acrylate,2,4-bis(4-dibenzofuranyl)-1-phenyl (meth)acrylate,2,4-bis(4-dibenzothiophenyl)-1-phenyl (meth)acrylate,4-methyl-2,6-bis(1-thianthrenyl)-1-phenyl (meth)acrylate,4-methyl-2,6-bis(2-benzothiophenyl)-1-phenyl (meth)acrylate,4-methyl-2,6-bis(3-benzothiophenyl)-1-phenyl (meth)acrylate,4-methyl-2,6-bis(4-dibenzofuranyl)-1-phenyl (meth)acrylate, and4-methyl-2,6-bis(4-dibenzothiophenyl)-1-phenyl (meth) acrylate.

The design flexibility of these (meth)acrylic monomers used as component(c) is high, and they are industrially advantageous.

(Compound ii)

Another preferred example of the polymerizable monomer is compound iidescribed in Japanese Unexamined Patent Application Publication No.2016-222566 and represented by at least formula (1) below.

In formula (1), Q is a thiophene-containing ring sulfide groupoptionally having a substituent. G is an acryloyl or methacryloyl group.L is a direct bond or an (r+s)-valent linking group linking Q to G andoptionally having a substituent. r represents an integer of 1 or moreand 5 or less. s represents an integer of 1 or more and 5 or less.

Compound ii represented by formula (1) will be described.

<L in Formula (1)>

L is a direct bond or any (r+s)-valent linking group linking Q to G andoptionally having a substituent. To impart a high refractive index, L ispreferably a group derived from a cyclic compound. To impartcompatibility, L is preferably a group derived from an aliphaticcompound. These structures may be combined according to the intendedpurpose of the material, and a linking group selected from —O—, —S—,—CO—, —COO—, and —CONH— may be present between a C—C bond.

To increase the compatibility of the compound as a whole, L ispreferably an aliphatic hydrocarbon group having 2 to 18 carbon atomsand preferably 3 to 6 carbon atoms. To maintain high compatibility whilethe size of L with respect to the compound as a whole is maintainedsmall, L is preferably a chain or cyclic saturated hydrocarbon group andmore preferably a hydrocarbon group having a branched structure.

Examples of the linear saturated hydrocarbon group include a methylenegroup, an ethylene group, a n-propylene group, a cyclopropylene group, an-butylene group, a n-pentylene group, and a n-hexylene group. Examplesof the hydrocarbon group having a branched structure include anisopropylene group, an isobutylene group, a s-butylene group, at-butylene group, a 1,1-dimethyl-n-propylene group, a1,2-dimethyl-n-propylene group, a 2,2-dimethyl-n-propylene group, a1-ethyl-n-propylene group, a 1-methyl-n-butylene group, a2-methyl-n-butylene group, a 3-methyl-n-butylene group, a1,1,2-trimethyl-n-propylene group, a 1,2,2-trimethyl-n-propylene group,a 1-ethyl-2-methyl-n-propylene group, a 1,1-dimethyl-n-butylene group, a2,2-dimethyl-n-butylene group, a 3,3-dimethyl-n-butylene group, a1,2-dimethyl-n-butylene group, a 1,3-dimethyl-n-butylene group, a2,3-dimethyl-n-butylene group, a 1-ethyl-n-butylene group, a2-ethyl-n-butylene group, a 1-methyl-n-pentylene group, a2-methyl-n-pentylene group, a 3-methyl-n-pentylene group, and a4-methyl-n-pentylene group. Examples of the cyclic hydrocarbon groupinclude a cyclohexylene group and a cyclopentylene group.

To increase the overall refractive index of the compound, L ispreferably a 3 to 8-membered cyclic compound and more preferably a 5 to6-membered cyclic compound. The ring in L may have a monocyclicstructure or a condensed ring structure. The number of rings forming Lis 1 to 4, preferably 1 to 3, and still more preferably 1 to 2. It isnot always necessary that the rings forming L have aromaticity. However,to maintain a high refractive index while the size of L with respect tothe molecule as a whole is maintained small, it is preferable that Lincludes an unsaturated bond, and it is more preferable that L is anaromatic hydrocarbon cyclic group or an aromatic heterocyclic group. Toensure optical transparency during holographic recording andreproduction, it is preferable that coloration is small. From this pointof view, L is more preferably an aromatic hydrocarbon cyclic group.

Examples of the aromatic hydrocarbon ring included in L include aromatichydrocarbon rings having 6 to 14 carbon atoms and preferably 6 to 12carbon atoms such as a benzene ring, an indene ring, a naphthalene ring,an azulene ring, a fluorene ring, an acenaphthylene ring, an anthracenering, a phenanthrene ring, and a pyrene ring. From the viewpoint ofavoiding coloration and ensuring solubility, L is particularlypreferably an aromatic hydrocarbon ring having 6 to 10 carbon atoms suchas a benzene ring or a naphthalene ring.

When the ring forming L is an aromatic heterocyclic ring, no particularlimitation is imposed on the heteroatom, and an atom such as S, O, N, orP can be used. From the viewpoint of ensuring compatibility, S, O, and Natoms are preferred. From the viewpoint of avoiding coloration andensuring solubility, the number of heteroatoms per molecule ispreferably 1 to 3.

Specific examples of the aromatic heterocyclic ring forming L includearomatic heterocyclic rings having 2 to 18 carbon atoms and preferably 3to 6 carbon atoms such as a furan ring, a thiophene ring, a pyrrolering, an imidazole ring, a triazole ring, a pyrazole ring, a pyridinering, a pyrimidine ring, a pyrazine ring, a triazine ring, an oxazolering, a triazole ring, a benzofuran ring, a benzothiophene ring, anindole ring, an isoindole ring, a benzimidazole ring, a quinoline ring,an isoquinoline ring, a quinoxaline ring, a dibenzofuran ring, adibenzothiophene ring, a carbazole ring, a thianthrene ring, adibenzothioxane ring, a dibenzobenzothiophene ring, an oxazolidine ring,and a thiazolidine ring. To maintain a high refractive index while thesize of L with respect to the compound as a whole is maintained small, athiophene ring, a furan ring, and a pyrrole ring are preferred.

<Substituent Optionally Included in L>

L may further have a substituent. For example, in order to improve thesolubility, L may have a substituent such as an alkyl group, an alkoxygroup, an alkoxyalkyl group, an alkoxycarbonyl group, an alkoxyalkoxygroup, or an alkanoyloxy group. To increase the refractive index, L mayhave an aryl group, an alkylthio group, an alkylthioalkyl group, anaryloxy group, or an arylalkoxy group. To achieve economical synthesis,L is preferably unsubstituted.

The alkyl group is preferably a chain alkyl group having 1 to 4 carbonatoms, and specific examples thereof include a methyl group, an ethylgroup, a n-propyl group, an isopropyl group, a s-butyl group, anisobutyl group, and a t-butyl group.

The alkoxy group is preferably an alkoxy group having 1 to 4 carbonatoms, and specific examples thereof include a methoxy group and anethoxy group.

The alkoxyalkyl group is preferably an alkoxyalkyl group having 2 to 6carbon atoms, and specific examples thereof include a methoxymethylgroup, an ethoxymethyl group, a propoxymethyl group, a butoxymethylgroup, a methoxyethyl group, an ethoxyethyl group, a propoxyethyl group,and a butoxyethyl group.

The alkoxycarbonyl group is preferably an alkoxycarbonyl group having 2to 5 carbon atoms, and specific examples thereof include amethoxycarbonyl group, an ethoxycarbonyl group, a propoxycarbonyl group,and a butoxycarbonyl group.

The alkoxyalkoxy group is preferably an alkoxyalkoxy group having 3 to 6carbon atoms, and specific examples thereof include a methoxyethoxygroup, an ethoxyethoxy group, a propoxyethoxy group, and a butoxyethoxygroup.

The alkanoyloxy group is preferably an alkanoyloxy group having 2 to 5carbon atoms, and specific examples thereof include an acetoxy group, apropionoxy group, a butyloxy group, and a valeroxy group.

The aryl group is preferably a monocyclic or condensed polycyclic arylgroup having 6 to 14 carbon atoms, and specific examples thereof includea phenyl group, a naphthyl group, and an anthranyl group.

The alkylthio group is preferably an alkylthio group having 2 to 4carbon atoms, and specific examples thereof include a methylthio group,an ethylthio group, a n-propylthio group, and an isopropylthio group.

The alkylthioalkyl group is preferably an alkylthioalkyl group having 2to 4 carbon atoms, and specific examples thereof include amethylthiomethyl group, a methylthioethyl group, an ethylthiomethylgroup, and an ethylthioethyl group.

The aryloxy group is a monocyclic or condensed polycyclic aryloxy grouphaving 6 to 14 carbon atoms, and specific examples thereof include aphenoxy group.

The arylalkoxy group is an arylalkoxy group having 7 to 5 carbon atoms,and specific examples thereof include a benzyloxy group.

<Q in Formula (1)>

Q is a thiophene-containing ring sulfide group optionally having asubstituent and is preferably a thiophene-containing ring sulfide grouprepresented by the following formula (2) or (3).

In formulas (2) and (3), the sulfide group may be bonded to any positionin the thiophene-containing ring. Q is a group essential for increasingthe refractive index of the compound represented by formula (1), and itis preferable that Q itself has a structure with a high refractiveindex.

The refractive index of Q itself can be estimated by the groupcontribution method. The refractive index of a compound can be estimatedas follows. For each of the atomic groups forming the compound, itsmolecular refraction [R] and its molar volume V₀ are used to computen={(1+2[R]/V₀)/(1−[R]/V₀)}^(0.5). Then the n's are summed up.

In applications requiring transparency, Q is more preferably thedibenzothiophene ring represented by formula (2) because its lightabsorption wavelength is on a shorter wavelength side and coloration byvisible light is small.

It is preferable that the molecular weight of Q is somewhat large inorder to reduce the rate of shrinkage due to crosslinking duringirradiation with light. The molecular weight of the moiety Q isgenerally 50 or more and preferably 70 or more. From the viewpoint ofensuring the mobility during optical recording, the molecular weight ofthe moiety Q is generally 300 or less and preferably 250 or less.

<Substituent Optionally Included in Q>

Q may optionally have a substituent. No particular limitation is imposedon the substituent optionally included in Q so long as it does not causea reduction in compatibility and a reduction in refractive index.Specific examples of such a substituent include alkylthio groups having1 to 3 carbon atoms such as a methylthio group and alkylthioalkyl groupshaving 2 to 6 carbon atoms such as a methylthiomethyl group.

<G in Formula (1)>

G represents an acryloyl group or a methacryloyl group.

<Bonding Position of Q to L>

Supposes that L is a cyclic group. Then, when r is 1, a preferredsubstitution position of Q on L is arbitrary. When r is 2 or more, it ispreferable that Qs are not adjacent to each other because a highsynthesis yield is obtained.

Suppose that L is a heteroaryl ring group. It is preferable that, in astructure in which L and Q are bonded to each other, partial structuresof L and Q that are bonded directly to each other contain no heteroatom.In other words, it is preferable that, in the structure in which L and Qare bonded to each other, heteroatom-containing partial structures of Land Q are not bonded directly to each other. A structure in which theheteroatom-containing partial structures of L and Q are bonded directlyto each other is not preferred because the structure tends to exhibitabsorption in the visible range and it is highly possible that theresulting coloration interferes with light transmission during recordingand reproduction.

<r and s>

r is an integer of 1 or more and 5 or less. When r is 2 or more, theplurality of Qs may be the same or different. To obtain goodcompatibility with a solvent and the matrix, r is preferably 1 to 3 andmore preferably 2 or 3.

s represents an integer of 1 or more and 5 or less. To reduce shrinkagedue to photocuring, s is preferably 1.

<Exemplary Compounds>

Specific examples of compound ii are exemplified below. However,compound ii is not limited to the compounds exemplified below.

As shown by the above structural formulas, examples of the compound iiinclude S-4-dibenzothiophenyl thioacrylate, S-7-benzothiophenylthioacrylate, S-(1,3-diphenyl)-4-dibenzothiophenyl thioacrylate,S-(1,3-diphenyl)-7-benzothiophenyl thioacrylate,6-phenyl-2,4-bis(4-dibenzothiophenyl)-1-phenyl acrylate,6-phenyl-2,4-bis(7-benzothiophenyl)-1-phenyl acrylate,7-(3-(4-dibenzothiophenyl)-2,6-dioxa-[3,3,0]bicyclooctyl) acrylate,7-(3-(7-benzothiophenyl)-2,6-dioxa-[3,3,0]bicyclooctyl) acrylate,2,4-bis(4-dibenzothiophenyl)-1-cyclohexyl acrylate,2,4-bis(7-benzothiophenyl)-1-cyclohexyl acrylate,3,3-bis(4-dibenzothiophenylthio)methylphenyl acrylate,3,3-bis(7-benzothiophenylthio)methylphenyl acrylate,4-(1,2-bis(4-dibenzothiophenylthio)ethylphenyl acrylate,4-(1,2-bis(7-benzothiophenylthio)ethylphenyl acrylate,2,3-bis(4-dibenzothiophenylthio) propyl acrylate,2,3-bis(7-benzothiophenylthio)propyl acrylate,2-(4-dibenzothiophenylthio)ethyl acrylate,2-(7-benzothiophenylthio)ethyl acrylate,1,3-bis(4-dibenzothiophenylthio)-2-propyl acrylate,1,3-bis(7-benzothiophenylthio)-2-propyl acrylate, and2,2-bis(4-dibenzothiophenylthiomethyl)-3-(4-dibenzothiophenylthio)propylacrylate.

The holographic recording medium composition of the present inventionmay contain, as component (c), only one of compound i and compound ii ortwo or more of them.

(Molar Absorption Coefficient of Polymerizable Monomer)

Preferably, the molar absorption coefficient of the polymerizablemonomer at the holographic recording wavelength is 100 L·mol⁻¹·cm⁻¹ orless. When the molar absorption coefficient is 100 L·mol⁻¹·cm⁻¹ or less,a reduction in the transmittance of the medium is prevented, andsufficient diffraction efficiency per thickness can be obtained.

<Component (d)>

The photopolymerization initiator generates cations, anions, or radicalsunder light to initiate a chemical reaction and contributes topolymerization of the above-described polymerizable monomer. Noparticular limitation is imposed on the type of photopolymerizationinitiator, and an appropriate photopolymerization initiator may beselected according to the type of polymerizable monomer.

The photopolymerization initiator used may be an oxime ester-basedphotopolymerization initiator exemplified below or an additionalphotopolymerization initiator other than the oxime ester-basedphotopolymerization initiator. It is preferable to use at least one ormore oxime ester-based photopolymerization initiators.

In this case, the oxime ester-based photopolymerization initiator isused in an amount of preferably 0.003% by weight or more, morepreferably 0.03% by weight or more, still more preferably 0.05% byweight or more, and particularly preferably 0.1 to 100% by weight basedon the total amount of the photopolymerization initiators.

Examples of the additional photopolymerization initiator includecationic photopolymerization initiators described later, anionicphotopolymerization initiators described later, and radicalphotopolymerization initiators described later. Among these additionalphotopolymerization initiators, radical photopolymerization initiatorsare preferably used because the matrix forming reaction is unlikely tobe inhibited. In particular, a phosphine oxide compound is morepreferred.

The photopolymerization initiator is more preferably a compound having amolar absorption coefficient at the recording wavelength of 1000L·mol⁻¹·cm⁻¹ or less. When the molar absorption coefficient is 1000L·mol⁻¹·cm⁻¹ or less, a reduction in transmittance of the holographicrecording medium at the recording wavelength that occurs when thephotopolymerization initiator is mixed in an amount enough to obtainsufficient diffraction efficiency can be prevented.

(Oxime Ester-Based Photopolymerization Initiator)

It is only necessary that the oxime ester-based photopolymerizationinitiator have —C═N—O— in part of its structure. In particular, acompound represented by (formula d) or (formula f) is preferred becausegood recording sensitivity is obtained.

In (formula d), X represents: a single bond; or a divalent groupselected from the group consisting of an alkylene group having 1 to 20carbon atoms and optionally having a substituent, an alkenylene group—(CH═CH)_(n)-optionally having a substituent, an alkynylene group—(C≡C)_(n)— optionally having a substituent, and combinations thereof (nis an integer of 1 or more and 5 or less).

R² represents a monovalent organic group having an aromatic ring and/ora heteroaromatic ring.

R³ represents a hydrogen atom or a group selected from the groupconsisting of an alkylthio group having 1 to 12 carbon atoms, analkoxycarbonyl group having 2 to 12 carbon atoms, an alkenyloxycarbonylgroup having 3 to 12 carbon atoms, an alkynyloxycarbonyl group having 3to 12 carbon atoms, an aryloxycarbonyl group having 7 to 12 carbonatoms, a heteroaryloxycarbonyl group having 3 to 12 carbon atoms, analkylthiocarbonyl group having 2 to 12 carbon atoms, analkenylthiocarbonyl group having 3 to 12 carbon atoms, analkynylthiocarbonyl group having 3 to 12 carbon atoms, anarylthiocarbonyl group having 7 to 12 carbon atoms, aheteroarylthiocarbonyl group having 3 to 12 carbon atoms, analkylthioalkoxy group, —O—N═CR³²R³³, —N(OR³⁴)—CO—R³³, and a grouprepresented by (formula e) below (R³² and R³³ each represent anoptionally substituted alkyl group having 1 to 12 carbon atoms and maybe different from each other, and R³⁴ and R³⁵ each represent anoptionally substituted alkyl group having 1 to 12 carbon atoms and maybe different from each other), each of these groups optionally having asubstituent.

In (formula e), R³⁰ and R³¹ each independently represent an optionallysubstituted alkyl group having 1 to 12 carbon atoms.

R⁴ represents a group selected from the group consisting of anoptionally substituted alkanoyl group having 2 to 12 carbon atoms, anoptionally substituted alkenoyl group having 3 to 25 carbon atoms, anoptionally substituted cycloalkanoyl group having 3 to 8 carbon atoms,an optionally substituted aryloyl group having 7 to 20 carbon atoms, anoptionally substituted heteroaryloyl group having 3 to 20 carbon atoms,an optionally substituted alkoxycarbonyl group having 2 to 10 carbonatoms, and an optionally substituted aryloxycarbonyl group having 7 to20 carbon atoms.

In (formula f), R⁵ represents a hydrogen atom, an alkyl group having 1to 20 carbon atoms and optionally having a substituent, an alkenyl grouphaving 2 to 25 carbon atoms and optionally having a substituent, aheteroaryl group having 3 to 20 carbon atoms and optionally having asubstituent, a heteroarylalkyl group having 4 to 25 carbon atoms andoptionally having a substituent, or a group bonded to Y¹ or Z to form aring.

R⁶ represents an alkanoyl group having 2 to 20 carbon atoms, an alkenoylgroup having 3 to 25 carbon atoms, a cycloalkanoyl group having 4 to 8carbon atoms, an aryloyl group having 7 to 20 carbon atoms, analkoxycarbonyl group having 2 to 10 carbon atoms, an aryloxycarbonylgroup having 7 to 20 carbon atoms, a heteroaryl group having 2 to 20carbon atoms, a heteroaryloyl group having 3 to 20 carbon atoms, or analkylaminocarbonyl group having 2 to 20 carbon atoms, each of thesegroups optionally having a substituent.

Y¹ represents a divalent aromatic hydrocarbon group and/or a divalentheteroaromatic group optionally having a substituent and formed bycondensation of two or more rings.

Z represents an aromatic group optionally having a substituent.

Specific examples of the oxime ester-based photopolymerization initiatorinclude1-(9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl)-1-(O-benzoyloxime)glutaricacid methyl ester,1-(9-ethyl-6-cyclohexanoyl-9H-carbazol-3-yl)-1-(O-acetyloxime)glutaricacid methyl ester,1-(9-ethyl-6-diphenylamino-9H-carbazol-3-yl)-1-(O-acetyloxime)hexane,1-(9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl)-1-(O-acetyloxime)ethanone,1-(9-ethyl-6-benzoyl-9H-carbazol-3-yl)-1-(O-acetyloxime)ethanone,1-(9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl)-1-(O-chloroacetyloxime)glutaricacid methyl ester,1-(9-ethyl-9H-carbazol-3-yl)-1-(O-acetyloxime)-3,3-dimethylbutane acid,1-(9-ethyl-6-(2-methylbenzoyl)-9H-carbazol-3-yl)-1-(O-acetyloxime)glutaricacid ethyl ester, 1-(4-(phenylthio)-2-(O-benzoyloxime))-1,2-octanedione,and compounds described in Japanese Unexamined Patent ApplicationPublication No. 2010-8713 and Japanese Unexamined Patent ApplicationPublication No. 2009-271502.

Among the compounds represented by (formula d) and (formula f) above,compounds having a ketoxime structure are more preferred.

(Preferred Oxime Ester-Based Photopolymerization Initiators)

Among the above oxime ester-based photopolymerization initiators, acompound represented by formula (4) below (hereinafter may be referredto as “compound (4)”) is particularly preferably used.

In formula (4), R²¹ represents an alkyl group. R²² represents an alkylgroup, an aryl group, or an aralkyl group. R²³ represents a —(CH₂)_(m)—group. m represents an integer of 1 or more and 6 or less. R²⁴represents a hydrogen atom or any substituent. R²⁵ represents anysubstituent having no multiple bond conjugated with a carbonyl groupbonded to R²⁵.

R²⁵ in formula (4) has a structure that is not conjugated with acarbonyl group, and therefore one advantage of compound (4) is that areduction in transmittance does not occur even when the holographicrecording medium is irradiated with light after recording. Otheradvantages of compound (4) are that, even after repetitions ofreproduction, the transfer rate of the holographic recording medium usedas a memory is not reduced and that, even when the holographic recordingmedium is used outdoors for light guide plates for AR glasses, nocoloration occurs.

The groups included in formula (4) will be described in detail.

<R²¹>

R²¹ represents an alkyl group. If R²¹ is a group such as an aromaticring group other than alkyl groups, the sp² characteristics of anitrogen atom bonded to R²¹ increase, and therefore conjugation from thecarbazole ring in formula (4) to R²¹ is extended, so that light emissionfrom a decomposed product after light irradiation occurs. Therefore, R²¹is preferably an alkyl group having no conjugated system.

Examples of the alkyl group denoted as R²¹ include linear, branched, andcyclic alkyl groups such as a methyl group, an ethyl group, a propylgroup, an isopropyl group, a cyclopropyl group, a butyl group, anisobutyl group, a sec-butyl group, a tert-butyl group, a pentyl group, acyclopentyl group, a hexyl group, a cyclohexyl group, a heptyl group, anoctyl group, a 2-ethylhexyl group, a 3,5,5-trimethylhexyl group, a decylgroup, a dodecyl group, a cyclopropyl group, a cyclobutyl group, acyclopentyl group, a cyclohexyl group, a cyclopropylmethyl group, acyclohexylmethyl group, and a 4-butylmethylcyclohexyl group. In thesealkyl groups, the number of carbon atoms is preferably 1 or more andmore preferably 2 or more and is preferably 18 or less and morepreferably 8 or less. In particular, a linear alkyl group having 1 to 4carbon atoms is preferred in terms of the crystallinity of compound (4).

The alkyl group denoted as R²¹ optionally has a substituent. Examples ofthe optional substituent include alkoxy groups, dialkylamino groups,halogen atoms, aromatic ring groups, and heterocyclic groups. Of these,alkoxy groups are preferred in terms of the ease of adjusting thesolubility of the compound.

Examples of the preferred alkoxy groups include a methoxy group, anethoxy group, a n-propoxy group, an isopropoxy group, a cyclopentylgroup, and a cyclohexyl group.

An alkyl group substituted with a fluorine atom has increased waterrepellency and is preferred in terms of improvement in the waterresistance of the compound.

<R²²>

R²² represents an alkyl group, an aryl group, or an aralkyl group. R²²is a radical site to be generated when the photopolymerization initiatoris irradiated with light and is preferably an alkyl group having a smallmolecular weight from the viewpoint of the mobility of the radical inthe holographic recording medium composition.

Specific examples of the alkyl group denoted as R²² are the same asthose described above for R²¹, and specific examples of the optionalsubstituent for R²² are also the same as those described for R²¹. Inthese alkyl groups, the number of carbon atoms is preferably 1 or moreand 4 or less and more preferably 2 or less in terms of the mobility ofradicals.

From the viewpoint of improving the water resistance of compound (4),R²² is preferably an aryl group.

Specific examples of the aryl group denoted as R²² include a phenylgroup, a naphthyl group, an anthryl group, and an indenyl group. Fromthe viewpoint of mobility of radicals, a phenyl group is preferred.

Specific examples of the aralkyl group denoted as R²² include a benzylgroup, a phenethyl group, and a naphthylmethyl group. Radicals such asbenzyl and naphthylmethyl are preferred because unique reactivity isexpected.

Examples of a substituent on the aryl group denoted as R²² or on thearyl moiety of the aralkyl group denoted as R²² include alkyl groups,halogen atoms, and alkoxy groups. From the viewpoint of the mobility ofradicals, R²² is preferably unsubstituted.

<R²³ and m>

R²³ represents a —(CH₂)_(m)— group. When R²³ is a substituentconjugatable with the carbazole ring in formula (4), the conjugationfrom the carbazole ring is extended to R²³, so that the absorptionwavelength of the photopolymerization initiator is shifted to the longerwavelength side. Therefore, R²³ is preferably the —(CH₂)_(m)— grouphaving no conjugated system.

m represents an integer of 1 or more and 6 or less. In particular, m ispreferably 2 or more and 4 or less. When m is in the above preferredrange, the electronical effect of substituent R²⁴ on R²³ tends to beeasily reflected.

<R²⁴>

R²⁴ represents a hydrogen atom or any substituent. No particularlimitation is imposed on the above substituent so long as the effects ofthe invention are not impaired. Example of the substituent include alkylgroups, alkoxycarbonyl groups, monoalkylaminocarbonyl groups,dialkylaminocarbonyl groups, aromatic ring groups, and heterocyclicgroups.

Of these, R²⁴ is preferably an alkyl group, an alkoxycarbonyl group, anaromatic ring group, or a heterocyclic group in terms of availability ofraw materials and ease of synthesis. R²⁴ is particularly preferably analkoxycarbonyl group in terms of obtaining an electronic effect.

Specific examples of the alkyl group denoted as R²⁴ are the same asthose described for R²¹, and specific example of the optionalsubstituent for R²⁴ are the same as those described for R²¹. In thesegroups, the number of carbon atoms is preferably 1 or more and 8 or lessand more preferably 4 or less in terms of adjusting the solubility ofthe compound.

Examples of the alkoxycarbonyl group denoted as R²⁴ include amethoxycarbonyl group, an ethoxycarbonyl group, and anisopropoxycarbonyl group. The number of carbon atoms in thealkoxycarbonyl group is preferably 2 or more and 19 or less and morepreferably 7 or less in terms of adjusting the solubility of thecompound.

The alkoxycarbonyl group optionally has a substituent. Examples of theoptional substituent include alkoxy groups, dialkylamino groups, andhalogen atoms. Of these, alkoxy groups are preferred in terms of ease ofadjusting the solubility.

Specific examples of the alkyl group moieties of themonoalkylaminocarbonyl and dialkylaminocarbonyl groups denoted as R²⁴are the same as those described above for the alkyl group denoted asR²¹, and specific example of the optional substituent for R²⁴ are thesame as those described for R²¹. In particular, the number of carbonatoms in the alkyl group moiety bonded to the amino group is preferably1 or more and 8 or less and more preferably 4 or less. When the numberof carbon atoms in the alkyl group moiety bonded to the amino group iswithin the above preferred range, solubility tends to be obtained.

Examples of the aromatic ring group denoted as R²⁴ include monocyclicrings and condensed rings such as a phenyl group, a naphthyl group, andan anthryl group. In these aromatic ring groups, the number of carbonatoms is 6 or more and is preferably 20 or less and more preferably 10or less in terms of the solubility of the compound.

The aromatic ring group optionally has a substituent. Examples of theoptional substituent include alkyl groups, alkoxy groups, dialkylaminogroups, and halogen atoms. Of these, alkyl groups are preferred in termsof ease of adjusting the solubility.

Preferred examples of the alkyl group include a methyl group, an ethylgroup, an isopropyl group, a cyclohexyl group, an isobutyl group, a2-ethylhexyl group, and a n-octyl group.

The heterocyclic group denoted as R²⁴ is preferably a group having atleast a heterocyclic structure having 1 or more heteroatoms in its ring.No particular limitation is imposed on the heteroatoms included in theheterocyclic group, and the heteroatoms may be atoms such as S, O, N,and P. In terms of availability of raw materials and ease of synthesis,atoms such as S, O, and N are preferred.

Specific examples of the heterocyclic group denoted as R²⁴ includemonocyclic rings and condensed rings such as a thienyl group, a furylgroup, a pyranyl group, a pyrrolyl group, a pyridyl group, a pyrazolylgroup, a thiazolyl group, a thiadiazolyl group, a quinolyl group, adibenzothiophenyl group, and a benzothiazolyl group. In theseheterocyclic groups, the number of carbon atoms is 1 or more and morepreferably 2 or more and is preferably 10 or less and more preferably 5or less in terms of availability of raw materials and ease of synthesis.

The heterocyclic group optionally has a substituent. Examples of theoptional substituent include alkyl groups, alkoxy groups, dialkylaminogroups, and halogen atoms. Of these, alkyl groups are preferred in termsof ease of adjusting the solubility.

Preferred examples of the alkyl group include a methyl group, an ethylgroup, an isopropyl group, a cyclohexyl group, an isobutyl group, a2-ethylhexyl group, and a n-octyl group.

<R²⁵>

R²⁵ represents any substituent that does not have a multiple bondconjugated with the carbonyl group bonded to R²⁵. When such asubstituent is used, no conjugated system extends beyond the carbazolering. In this case, the length of the conjugated system in adecomposition product generated after light irradiation is not extended,so that the absorption wavelength of the decomposition product is notshifted to the long wavelength side. Therefore, an absorption reducingeffect after recording is obtained.

Example of R²⁵ include alkyl groups, alkoxy groups, dialkylamino groups,alkylsulfonyl groups, and dialkylaminosulfonyl groups. Of these, alkylgroups are preferred in terms of the stability of the compound.

Specific examples of the alkyl group denoted as R²⁵ are the same asthose described for R²¹, and specific example of the optionalsubstituent for R²⁵ are the same as those described for R²¹. In terms ofease of production of the compound, the alkyl group is preferablybranched, and the number of carbon atoms thereof is preferably 1 or moreand more preferably 2 or more and is preferably 18 or less and morepreferably 10 or less. Particularly preferred specific examples includea cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, a cycloheptyl group, an isopropyl group, a tert-butylgroup, an adamantyl group, and a 1-ethylpentyl group.

Specific examples of the alkyl moieties of the alkoxy, dialkylamino,alkylsulfonyl, and dialkylaminosulfonyl groups denoted as R²⁵ are thesame as those described for R²¹, and specific example of the optionalsubstituent for R²⁵ are the same as those described for R²¹. Inparticular, the number of carbon atoms is preferably 3 or more and 18 orless and more preferably 8 or less in terms of the crystallinity of thecompound.

The absorption wavelength range of compound (4) is preferably 340 nm ormore and more preferably 350 nm or more and is preferably 700 nm or lessand more preferably 650 nm or less. For example, when the light sourceis a blue laser, it is preferable that compound (4) has absorption in atleast 350 to 430 nm. When a green laser is used, it is preferable thatcompound (4) has absorption in at least 500 to 550 nm. When theabsorption wavelength range differs from the above range, it isdifficult to use the energy of the irradiation light efficiently for thephotopolymerization reaction, and therefore, the sensitivity tends todecrease.

The molar absorption coefficient of compound (4) at the holographicrecording wavelength is preferably 10 L·mol⁻¹·cm⁻¹ or more and morepreferably 50 L·mol⁻¹·cm⁻¹ or more. The molar absorption coefficient ispreferably 20000 L·mol⁻¹·cm⁻¹ or less and more preferably 10000L·mol⁻¹·cm⁻¹ or less. When the molar absorption coefficient is withinthe above range, effective recording sensitivity can be obtained, and anexcessive reduction in the transmittance of the medium can be prevented,so that sufficient diffraction efficiency per thickness tends to beobtained.

The solubility of compound (4) in component (a) and component (b) underthe conditions of 25° C. and 1 atm is preferably 0.01% by weight or moreand more preferably 0.1% by weight or more.

Among commonly used oxime ester-based initiators, initiators having theabsorption maximum within the above-described absorption wavelengthrange often have poor solubility in materials forming components (a) and(b) such as isocyanates and polyols and in component (c) such as(meth)acrylates. However, compound (4) has good solubility in components(a) to (c) such as isocyanates, polyols, and (meth)acrylates andtherefore can be preferably used for the holographic recording mediumcomposition.

Specific examples of the photopolymerization initiator represented bycompound (4) are shown below but are not limited thereto.

No particular limitation is imposed on the method for synthesizingcompound (4), and compound (4) can be produced using a combination ofcommonly used synthesis methods. Specific examples include a methoddescribed in International Publication No. 2009/131189.

For example, as shown in a chemical formula below, an acyl group havingR²⁵ and an acyl group having R²⁴ and R²³ can be introduced into acarbazole ring through a Friedel-Crafts reaction. The Friedel-Craftsreaction is described in, for example,

Andrew Streitwieser. Jr. et. al, Introduction to Organic Chemistry,Macmillan Publishing Company, NewYork, P 652-653 and

Bradford p. Mundy et. al., Name Reactions and Reagents in OrganicSynthesis, A Wiley-Interscience Publication, P 82-83.

Methylene can be nitrosated by a method using nitrous acid or a nitrite.

A hydroxy group can be acylated by a method using a corresponding acidhalide or anhydride and a base.

The nitrosation and acylation in the following chemical formula aredescribed in, for example, Japanese Unexamined Patent ApplicationPublication (Translation of PCT Application) No. 2004-534797 and OrganicReaction Volume VII, KRIEGER PUBLISHING COMPANY MALABAR, Fla., Chapter6.

R²¹ to R²⁵ above are the same as R²¹ to R²⁵ in formula (4).

(Cationic Photopolymerization Initiator)

The cationic photopolymerization initiator used may be any knowncationic photopolymerization initiator. Examples of the cationicphotopolymerization initiator include aromatic onium salts. Specificexamples of the aromatic onium salts include compounds containing ananionic component such as SbF₆ ⁻, BF₄ ⁻, AsF₆ ⁻, PF₆ ⁻, CF₃SO₃ ⁻, orB(C₆F₅)O₄ ⁻ and an aromatic cationic component containing an atom suchas iodine, sulfur, nitrogen, or phosphorus. Of these, diaryliodoniumsalts, triarylsulfonium salts, etc. are preferred. Any one of the aboveexemplified cationic photopolymerization initiators may be used alone,or any combination of two or more of them may be used at any ratio.

(Anionic Photopolymerization Initiator)

The anionic photopolymerization initiator used may be any known anionicphotopolymerization initiator. Examples of the anionicphotopolymerization initiator include amines. Examples of the aminesinclude: amino group-containing compounds such as dimethylbenzylamine,dimethylaminomethylphenol, 1,8-diazabicyclo[5.4.0]undecene-7, andderivatives thereof; and imidazole compounds such as imidazole,2-methylimidazole, 2-ethyl-4-methylimidazole, and derivatives thereof.Any one of the above-exemplified anionic photopolymerization initiatorsmay be used alone, or any combination of two or more of them may be usedat any ratio.

(Radical Photopolymerization Initiator)

The radical photopolymerization initiator used may be any known radicalphotopolymerization initiator. Examples of the radicalphotopolymerization initiator used include phosphine oxide compounds,azo compounds, azide compounds, organic peroxides, organic boronic acidsalts, onium salts, bisimidazole derivatives, titanocene compounds,iodonium salts, organic thiol compounds, and halogenated hydrocarbonderivatives. Any one of the above-exemplified radicalphotopolymerization initiators may be used alone, or any combination oftwo or more of them may be used at any ratio. Of these, phosphine oxidecompounds are preferred as described above.

No particular limitation is imposed on the type of phosphine oxidecompound so long as the object and effects of the invention are notimpaired. In particular, the phosphine oxide compound is preferably anacylphosphine oxide compound.

Examples of the acylphosphine oxide compound include monoacylphosphineoxide represented by (formula b) below and diacylphosphine oxiderepresented by (formula c) below. Any one of them may be used alone, orany combination of two or more of them may be used at any ratio.

In (formula b), R⁷ represents: an alkyl group having 1 to 18 carbonatoms; an alkyl group having 1 to 4 carbon atoms, a cycloalkyl grouphaving 5 to 8 carbon atoms, a phenylalkyl group having 7 to 9 carbonatoms, a phenyl group, a naphthyl group, or a biphenyl group, each ofthese groups being substituted with a halogen or an alkoxy group having1 to 6 carbon atoms; a phenyl group, a naphthyl group, or a biphenylgroup, each of these groups being substituted with at least one selectedfrom the group consisting of halogens, alkyl groups having 1 to 12carbon atoms, and alkoxy groups having 1 to 12 carbon atoms; or amonovalent 5- or 6-membered heterocyclic ring containing N, O, or S.

R⁸ represents: a phenyl group; a naphthyl group; a biphenyl group; aphenyl group, a naphthyl group, or a biphenyl group, each of thesegroups being substituted with at least one selected from the groupconsisting of halogens, alkyl groups having 1 to 12 carbon atoms, andalkoxy groups having 1 to 12 carbon atoms; a monovalent 5- or 6-memberedheterocyclic ring containing N, O, or S; an alkoxy group having 1 to 18carbon atoms; a phenoxy group; or a phenoxy group, a benzyloxy group, ora cyclohexyloxy group, each of these groups being substituted with ahalogen, an alkyl group having 1 to 4 carbon atoms, or an alkoxy grouphaving 1 to 4 carbon atoms. R⁸ and R⁷ may form a ring together with aphosphorus atom.

R⁹ is: an alkyl group having 1 to 18 carbon atoms; an alkyl group having1 to 4 carbon atoms, a cycloalkyl group having 5 to 8 carbon atoms, aphenylalkyl group having 7 to 9 carbon atoms, a phenyl group, a naphthylgroup, or a biphenyl group, each of these groups being substituted witha halogen or an alkoxy group having 1 to 6 carbon atoms; a phenyl group,a naphthyl group, or a biphenyl group, each of these groups beingsubstituted with at least one selected from the group consisting ofhalogens, alkyl groups having 1 to 12 carbon atoms, and alkoxy groupshaving 1 to 12 carbon atoms; a monovalent 5- or 6-membered heterocyclicring containing N, O, or S; or a group represented by (formula b-1)below.

In (formula b-1), B¹ represents: an alkylene group having 2 to 8 carbonatoms or a cyclohexylene group having 2 to 8 carbon atoms; or aphenylene group or a biphenylene group, each of these groups beingunsubstituted or substituted with a halogen, an alkyl group having 1 to4 carbon atoms, or an alkoxy group having 1 to 4 carbon atoms.

R^(7′) and R^(8′) are the same groups as those described for R⁷ and R⁸,respectively, in (formula b) above.

In the molecules represented by (formula b) and (formula b-1), R⁷ andR^(7′) may be the same or different, and R⁸ and R^(8′) may be the sameor different.

In (formula c), R¹⁰ represents: an alkyl group having 1 to 18 carbonatoms; an alkyl group having 1 to 4 carbon atoms, a cycloalkyl grouphaving 5 to 8 carbon atoms, a phenylalkyl group having 7 to 9 carbonatoms, a phenyl group, a naphthyl group, or a biphenyl group, each ofthese groups being substituted with a halogen or an alkoxy group having1 to 6 carbon atoms; a phenyl group, a naphthyl group, or a biphenylgroup, each of these groups being substituted with at least one selectedfrom the group consisting of halogens, alkyl groups having 1 to 12carbon atoms, and alkoxy groups having 1 to 12 carbon atoms; amonovalent 5- or 6-membered heterocyclic ring containing N, O, or S, analkoxy group having 1 to 18 carbon atoms, or a phenoxy group; or aphenoxy group, a benzyloxy group, or a cyclohexyloxy group, each ofthese groups being substituted with a halogen, an alkyl group having 1to 4 carbon atoms, or an alkoxy group having 1 to 4 carbon atoms.

R¹¹ and R¹² each independently represent: an alkyl group having 1 to 18carbon atoms; an alkyl group having 1 to 4 carbon atoms, a cycloalkylgroup having 5 to 8 carbon atoms, a phenylalkyl group having 7 to 9carbon atoms, a phenyl group, a naphthyl group, or a biphenyl group,each of these groups being substituted with a halogen or an alkoxy grouphaving 1 to 6 carbon atoms; a phenyl group, a naphthyl group, or abiphenyl group, each of these groups being substituted with at least oneselected from the group consisting of halogens, alkyl groups having 1 to12 carbon atoms, and alkoxy groups having 1 to 12 carbon atoms; or amonovalent 5- or 6-membered heterocyclic ring containing N, O, or S.

<Component (e)>

Component (e) is a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group. When the holographic recording medium composition of thepresent invention contains component (e), the isocyanate-reactivefunctional group in component (e) reacts with the isocyanate group incomponent (a), or the isocyanate group in component (e) reacts with theisocyanate-reactive functional group in component (b). In this case,component (e) is fixed to the resin matrix, and the recordingsensitivity is improved by the nitroxyl radical group contained incomponent (e), so that high M/# can be achieved.

Examples of the isocyanate-reactive functional group included incomponent (e) include the same groups as those for theisocyanate-reactive functional group included in component (b).

Components (e-1) to (e-4) used as component (e) in the present inventionwill be described.

In the following description, 9-azabicyclo[3.3.1]nonane N-oxylrepresented by structural formula (E-1) below is abbreviated as “ABNO.”

2-Azatricyclo[3.3.1.1^(3,7)]decane N-oxyl represented by structuralformula (E-2) below is abbreviated as “AZADO.”

2-Aazatricyclo[3.3.1.0^(5,8)]nonane N-oxyl represented by structuralformula (E-3) below is abbreviated as “nor-AZADO.”

Numerical values 1 to 10 in structural formulas (E-1) to (E-3) belowrepresent the positions of atoms.

In the description of substituents in formula (I), a numerical rangeplaced after “C” in a substituent represents the number of carbon atomsin the substituent. For example, “a C1-12 alkyl group” means “an alkylgroup having 1 to 12 carbon atoms.”

R^(A) in formulas (I) and (Ia) to (Ic) represents a substituent bondedto a carbon atom in a bicyclic ring structure or a tricyclic ringstructure, and one or two R^(A)s are bonded to each carbon atom in thebicyclic or tricyclic ring structure. For example, a compoundrepresented by formula (Ib) described later is the same as a compoundrepresented by formula (Ib′) below, and the same applies to formulas(I), (Ia), and (Ic).

<Component (e-1)>

Component (e-1) is a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group. This compound has a heterobicyclic ring structure or aheterotricyclic ring structure obtained by replacing a carbon atom in abicyclic ring structure or a tricyclic ring structure by the nitroxylradical group.

In the compound denoted as component (e-1), the isocyanate group or theisocyanate-reactive functional group is bonded directly or through anylinking group to a carbon atom included in a compound having theheterobicyclic ring structure or the heterotricyclic ring structureobtained by replacing a carbon atom in a bicyclic ring structure or atricyclic ring structure by the nitroxyl radical group.

Hereinafter, a partial structure of component (e-1) other than theisocyanate group or the isocyanate-reactive functional group may bereferred to as “a mother compound of component (e-1).” The same appliesto components (e-3) and (e-4) described later.

Examples of the mother compound of component (e-1) include, but notlimited to, the ABNO, AZADO, and nor-AZADO described above. Each of theABNO, AZADO, and nor-AZADO may have a substituent other than anisocyanate group and an isocyanate-reactive functional group at anyposition. Examples of the substituent other than an isocyanate group oran isocyanate-reactive functional group include a substituentrepresented by R^(A) in formula (I) for component (e-2) described later.The above substituent is preferably a halogen atom, an alkyl grouphaving 1 to 6 carbon atoms, a hydrogen atom, etc. When the mothercompound is ABNO, AZADO, or nor-AZADO, the substitution position of thesubstituent is preferably the 1-, 3-, 5-, or 7-position.

When bulky substituents are bonded to two carbon atoms covalently bondedto the nitrogen atom of the nitroxyl radical group (these carbon atomsmay be referred to as “adjacent carbon atoms”), the nitroxyl radicalgroup is sterically hindered, and the effects of the invention may notbe obtained. These adjacent carbon atoms may have substituents. However,preferably, only one of the adjacent carbon atoms has a substituent, andthe other one has a hydrogen atom. Preferably, each of the substituentson the adjacent carbon atoms is an alkyl group having 3 or less carbonatoms or a halogen atom.

Examples of the mother compound of component (e-1) include compoundsshown below. In the following exemplary formulas, “Ac” represents “anacetyl group.”

No particular limitation is imposed on the substitution position of theisocyanate group or the isocyanate-reactive functional group on themother compound of component (e-1). In AZADO, the substitution positionis preferably the 5- or 6-position. In ABNO, the substitution positionis preferably the 3- or 7-position. In nor-AZADO, the substitutionposition is preferably the 3- or 7-position.

When the isocyanate group or the isocyanate-reactive functional group isbonded to the mother compound of component (e-1) through a liking group,the linking group is an alkyl group having 1 to 6 carbon atoms, anarylene group such as a phenylene group, a peptide bond (—OCO—NH—), aurethane group (—OCO—NH—), or a group obtained by combining two or moreof them.

Specific examples of component (e-1) include3-hydroxy-9-azabicyclo[3.3.1]nonane N-oxyl (3-HO-ABNO) represented bystructural formula (e-1-1) below,5-hydroxy-2-azatricyclo[3.3.1.1^(3,7)]nonane N-oxyl (5-HO-AZADO)represented structural formula (e-1-2) below, and compounds representedstructural formulas (e-1-3) to (e-1-8) below.

<Component (e-2)>

Component (e-2) is a compound represented by formula (I) below.

wherein, in formula (I), C₁ and C₂ each represent a carbon atom; twoR^(X)s in formula (I) are each the same as R^(A) or are bonded togetherand represent a direct bond or a linking group, the direct bond or thelinking group covalently bridging the two carbon atoms C₁ and C₂; and,when R^(X) and R^(X) represent the linking group, the linking group isrepresented by —C(—R^(A))₂— or —C(—R^(A))₂—C(—R^(A))₂—,

wherein, in formula (I) and the linking group, each R^(A) represents asubstituent selected from a hydrogen atom, halogen atoms, C1-12 alkylgroups, a hydroxy group, hydroxy(C1-12 alkyl) groups, an amino group,amino(C1-12 alkyl) groups, an isocyanate group, and isocyanate(C1-12alkyl) groups; in formula (I) and the linking group, one or two R^(A)sare bonded to each carbon atom in a bicyclic ring structure or atricyclic ring structure; when the number of substituents on a carbonatom is two or more, the substituents may be the same or different; and,in formula (I) and the linking group, the plurality of R^(A)s may be thesame or different,

provided that at least one of R^(A)s in formula (I) and the linkinggroup is at least one substituent selected from a hydroxy group,hydroxy(C1-12 alkyl) groups, an amino group, amino(C1-12 alkyl) groups,and an isocyanate group.

Component (e-2) is particularly preferably a compound represented by anyof formulas (Ia) to (Ic) below.

wherein, in formulas (Ia) to (Ic), each R^(A) is the same as that informula (I).

Preferred and specific examples of component (e-2) include thoseexemplified for component (e-1).

<Component (e-3)>

Component (e-3) is a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group, and the nitroxyl radical group is not stericallyhindered. Since the nitroxyl radical group in component (e-3) is notsterically hindered, component (e-3) is highly effective as component(e).

Example of the structure in which the nitroxyl radical group is notsterically hindered include modes described in (e-3-1) to (e-3-3).

(e-3-1): a compound in which the number of alkyl groups such as methylgroups covalently bonded to respective two atoms covalently bonded tothe nitrogen atom of the nitroxyl radical group (these atoms are notlimited to carbon atoms and may be sulfur atoms. These two atoms may bethe same or different) is 0 or 1.

(e-3-2): a compound in which at least one of the two carbon atomscovalently bonded to the nitrogen atom of the nitroxyl radical group iscovalently bonded to at least one hydrogen or halogen atoms.

(e-3-3): a compound in which both of the two carbon atoms covalentlybonded to the nitrogen atom of the nitroxyl radical group are eachcovalently bonded to at least one hydrogen or halogen atoms.

In the mode denoted as (e-3-1), examples of the mother compound ofcomponent (e-3) include compounds below.

In the mode denoted as (e-3-2), examples of the mother compound ofcomponent (e-3) include compounds below.

In the mode denoted as (e-3-3), examples of the mother compound ofcomponent (e-3) include compounds below.

Preferred examples of the substitution position of the isocyanate groupor the isocyanate-reactive functional group on the mother compound ofcomponent (e-3) and preferred examples of the linking group between themother compound and the isocyanate group or the isocyanate-reactivefunctional group are the same as those for component (e-1).

<Component (e-4)>

Component (e-4) is a compound in which an isocyanate group or anisocyanate-reactive functional group is substituted on a mother compoundhaving a nitroxyl radical group, and the redox potential of the mothercompound having the nitroxyl radical group is 280 mV or less.

A compound with a low redox potential easily releases an electron, andits oxidized form is stable. Therefore, such a compound has excellentradical trapping performance and high reliability. When the redoxpotential of a mother compound is equal to or lower than the above upperlimit, a compound including the mother compound and an isocyanate groupor an isocyanate-reactive functional group substituted on the mothercompound directly or through a linking group also has a low redoxpotential and is highly effective as component (e).

Specific examples of the compound having a redox potential of 280 mV orless and having a nitroxyl radical group include compounds listed below.The redox potentials of these compounds are also shown.

Preferably, the mother compound of component (e-4) is any of thesecompounds.

Conventional TEMPO has a high redox potential as shown below. Therefore,the radical trapping performance of TEMPOL is low.

Preferred examples of the substitution position of the isocyanate groupor the isocyanate-reactive functional group on the mother compound ofcomponent (e-4) and preferred examples of the linking group between themother compound and the isocyanate group or the isocyanate-reactivefunctional group are the same as those for component (e-1).

The holographic recording medium composition of the present inventionmay contain, as component (e), only one of component (e-1) to component(e-4) or two or more of them.

<Component (f)>

Preferably, the holographic recording medium composition of the presentinvention further contains, as component (f), a curing catalyst thatfacilitates the reaction between the isocyanate group-containingcompound denoted as component (a) and the isocyanate-reactive functionalgroup-containing compound denoted as (b). Preferably, a bismuth-basedcatalyst serving as Lewis acid is used as the curing catalyst denoted ascomponent (f).

Examples of the bismuth-based catalyst include tris(2-ethylhexanoate)bismuth, tribenzoyloxy bismuth, bismuth triacetate, bismuthtris(dimethyldithiocarbamate), bismuth hydroxide, triphenylbismuth(V)bis(trichloroacetate), tris(4-methylphenyl)oxobismuth(V), andtriphenylbis(3-chlorobenzoyloxy) bismuth(V).

Of these, trivalent bismuth compounds are preferred in terms ofcatalytic activity, and bismuth carboxylate and a compound representedby a general formula Bi(OCOR)₃ (R is a linear or branched alkyl group, acycloalkyl group, or a substituted or unsubstituted aromatic group) aremore preferred. Any one of these bismuth-based catalysts may be usedalone, or any combination of two or more of them may be used at anyratio.

To control the reaction rate, an additional curing catalyst may be usedin combination with the bismuth-based catalyst. No particular limitationis imposed on the catalyst that can be used in combination with thebismuth-based catalyst so long as it does not depart from the spirit ofthe invention. To obtain the synergistic effect of the catalysts, it ispreferable to use a compound having an amino group in part of itsstructure. Examples of such a compound include amine compounds such astriethylamine (TEA), N,N-dimethylcyclohexylamine (DMEDA),N,N,N′,N′-tetramethylethylenediamine (TMEDA),N,N,N′,N′-tetramethylpropane-1,3-diamine (TMPDA),N,N,N′,N′-tetramethylhexane-1,6-diamine (TMHMDA),N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA),N,N,N′,N″,N″-pentamethyldipropylene-triamine (PMDPTA),triethylenediamine (TEDA), N,N′-dimethylpiperazine (DMP),N,-methyl,N′-(2dimethylamino)-ethylpiperazine (TMNAEP),N-methylmorpholine (NMMO), N.(N′,N′-dimethylaminoethyl)-morpholine(DMAEMO), bis(2-dimethylaminoethyl) ether (BDMEE), ethylene glycolbis(3-dimethyl)-aminopropyl ether (TMEGDA), and diisopropylethylamine(DIEA). Any one of these compounds may be used alone, or any combinationof two or more of them may be used at any ratio.

<Additional Components>

The holographic recording medium composition of the present inventionmay further contain additional components other than components (a) to(f) described above so long as they do not depart from the spirit of theinvention.

Examples of the additional components include: a solvent, a plasticizer,a dispersant, a leveling agent, an antifoaming agent, an adhesionpromoter, etc. that are used to prepare the recording layer of theholographic recording medium; and a chain transfer agent, apolymerization terminator, a compatibilizer, a reaction aid, asensitizer, an antioxidant, etc. that are used to control the recordingreaction. Any one of these components may be used alone, or anycombination of two or more of them may be used at any ratio.

Of these, it is preferable to use a chain transfer agent. In particular,a compound having a terpenoid skeleton is more preferably used. Thecompound having a terpenoid skeleton may be any compound having theterpenoid skeleton. Specific examples thereof include monoterpenes (suchas camphor, menthol, limonene, terpineol, geraniol, nerol, citronellol,terpinolene, and a, (3, and y-terpinenes); sesquiterpenes (such asfarnesol, nerolidol, and caryophyllene); diterpenes (such as abieticacid, taxol, pimaric acid, geranylgeraniol, and phytol); triterpenes(such as squalene); and carotenoids. Of these, terpinolene, α-terpinene,β-terpinene, γ-terpinene, etc. are particularly preferred. Any one ofthe above-exemplified terpenoid skeleton-containing compounds may beused alone, or any combination of two or more of them may be used at anyratio.

<Compositional Ratios of Components of Holographic Recording MediumComposition>

The contents of the components of the holographic recording mediumcomposition of the present invention are freely set so long as they donot depart from the spirit of the invention. However, preferably, thecontents of the components are within the following ranges.

The total content of components (a) and (b) in the holographic recordingmedium composition of the present invention is generally 0.1% by weightor more, preferably 10% by weight or more, and more preferably 35% byweight or more and is generally 99.9% by weight or less, preferably 99%by weight or less, and more preferably 98% by weight or less. When thetotal content is equal to or more than the above lower limit, therecording layer can be easily formed. When the total content is equal toor less than the above upper limit, the contents of other essentialcomponents can be ensured.

The ratio of the number of isocyanate-reactive functional groups incomponent (b) to the number of isocyanate groups in component (a) ispreferably 0.1 or more and more preferably 0.5 or more and is generally10.0 or less and preferably 2.0 or less. When this ratio is within theabove range, the number of unreacted functional groups is small, andstorage stability is improved.

The content of component (c) in the holographic recording mediumcomposition of the present invention is generally 0.1% by weight ormore, preferably 1% by weight or more, and more preferably 2% by weightor more and is generally 80% by weight or less, preferably 50% by weightor less, and more preferably 30% by weight or less. When the amount ofcomponent (c) is equal to or more than the above lower limit, sufficientdiffraction efficiency is obtained. When the amount is equal to or lessthan the above upper limit, the compatibility of the recording layer ismaintained.

The ratio of the amount of component (d) in the holographic recordingmedium composition of the present invention to the amount of component(c) is generally 0.1% by weight or more, preferably 0.2% by weight ormore, and more preferably 0.3% by weight or more and is generally 20% byweight or less, preferably 18% by weight or less, and more preferably16% by weight or less. When the ratio of component (d) is equal to ormore than the above lower limit, sufficient recording sensitivity isobtained. When the ratio is equal to or less than the above upper limit,a reduction in sensitivity caused by a bimolecular termination reactiondue to excessive generation of radicals can be prevented.

As for the content of component (e) in the holographic recording mediumcomposition of the present invention, the molar ratio of component (e)to component (d) (component (e)/component (d)) is generally 0.1 or more,particularly preferably 0.2 or more, and most preferably 0.3 or more andis generally 10 or less, particularly preferably 8 or less, and mostpreferably 6 or less.

When component (e)/component (d) is equal to or more than the abovelower limit, the effect of improving M/# due to the presence ofcomponent (e) can be effectively obtained. When the ratio is equal to orless than the above upper limit, a radical polymerization reactionproceeds during exposure to light performed for the purpose ofrecording. In this case, the degree of refractive index modulationnecessary to form a diffraction grating can be obtained, and sufficientrecording sensitivity can be obtained.

Preferably, the content of component (f) in the holographic recordingmedium composition of the present invention is determined inconsideration of the rate of the reaction between component (a) andcomponent (b). The content of component (f) is generally 5% by weight orless, preferably 4% by weight or less, and more preferably 1% by weightor less and is preferably 0.001% by weight or more.

It is only necessary that the total amount of components other thancomponents (a) to (f) in the holographic recording medium composition ofthe present invention be 30% by weight or less, and the total amount ispreferably 15% by weight or less and more preferably 5% by weight orless.

<Method for Producing Holographic Recording Medium Composition>

To produce the holographic recording medium composition of the presentinvention, components (a) to (e), preferably components (a) to (f), maybe freely combined and mixed in any order. In this case, additionalcomponents may be combined and mixed.

The holographic recording medium composition of the present inventioncan be produced by, for example, the following method, but the inventionis not limited thereto.

All the components other than component (b) and component (f) are mixed,and the mixture is used as solution A. Component (b) and component (f)are mixed, and the mixture is used as solution B. Preferably, thesesolutions are subjected to dehydration and deaeration. If dehydrationand deaeration are not performed or are insufficient, air bubbles may begenerated during production of the medium, and a uniform recording layermay not be obtained. During dehydration and deaeration, heating andevacuation may be performed so long as the components are not impaired.

Component (e) is a minority component. Therefore, to improve theuniformity and dispersibility of component (e) in the composition and toobtain chemical bonding with component (a) and component (b) reliably,it is preferable to produce a master batch for component (e) in advance.For example, when component (e) has an isocyanate-reactive functionalgroup, it is preferable that part, e.g., 10 to 90% by weight, ofcomponent (a) and component (e) are mixed to obtain a master batch. Thenthe master batch is mixed with solution A prepared by mixing all thecomponents other than components (b), (e), and (f) (as for component(a), the remainder thereof not used for the master batch) and solution Bprepared by mixing components (b) and (f). When component (e) has anisocyanate group, a master batch may be produced in advance by mixingcomponents (b) and (e) in a similar manner.

When component (e) has an isocyanate-reactive functional group, part,e.g., 10 to 90% by weight, of component (f) may be further mixed intothe master batch for component (e), and the remainder of component (f)may be mixed into solution B.

Solutions A and B or solutions A and B and the master batch forcomponent (e) are mixed immediately before molding. In this case, aconventional mixing technique may be used. When solutions A and B aremixed or solutions A and B and the master batch for component (e) aremixed, degassing may be optionally performed in order to remove residualgasses. Preferably, solutions A and B or solutions A and B and themaster batch for component (e) are subjected to a filtration operationseparately or after mixing in order to remove foreign substances andimpurities. More preferably, these solutions are filtrated separately.An isocyanate functional prepolymer prepared through a reaction betweenexcessive component (a) and component (b) may be used as component (a)′.An isocyanate-reactive prepolymer prepared through a reaction betweenexcessive component (b) and component (a) may be used as component (b)′.

[Cured Product for Holographic Recording Medium]

The cured product for a holographic recording medium of the presentinvention can be obtained by curing the holographic recording mediumcomposition of the present invention.

No particular limitation is imposed on the method for forming the curedproduct for a holographic recording medium of the present invention. Toproduce the holographic recording medium composition of the presentinvention, the above-described solutions A and B or the above-describedsolutions A and B and the master batch for component (e-1) are mixed.Then the mixture is cured to produce the cured product for a holographicrecording medium.

In this case, the mixture may be heated to 30 to 100° C. for 1 to 72hours in order to facilitate the curing reaction.

The cured product for a holographic recording medium of the presentinvention can be produced also by a recording layer forming method in amethod for producing the holographic recording medium of the presentinvention described later.

In the cured product for a holographic recording medium of the presentinvention that is produced by curing the holographic recording mediumcomposition of the present invention, the total content of the PG unitand the TMG unit derived from component (b) in the matrix formed fromcomponent (a) and component (b) is preferably 30% by weight or less,more preferably 27% by weight or less, and particularly preferably 25%by weight or less because of the same reason as that for the holographicrecording medium composition of the present invention. Most preferably,no PG unit and no TMG unit are contained.

Moreover, in the cured product for a holographic recording medium of thepresent invention, the content of the CL unit derived from component (b)in the matrix formed from component (a) and component (b) is 20% byweight or more, particularly preferably 25% by weight or more, and mostpreferably 30 to 70% by weight.

The contents of the PG, TMG, and CL units derived from component (b) inthe cured product for a holographic recording medium of the presentinvention can be quantified, for example, by subjecting the curedproduct for a holographic recording medium to alkaline hydrolysis andanalyzing the obtained polyol component using nuclear magnetic resonance(NMR) or gas chromatography mass spectrometry (GC/MS).

The presence of the compound denoted as component (e), having anisocyanate group or an isocyanate-reactive functional group, and furtherhaving a nitroxyl radical group in the cured product for a holographicrecording medium of the present invention can be examined using anelectron spin resonance (ESR) device.

[Stacked Body for Holographic Recording Medium]

By stacking the cured product for a holographic recording medium of thepresent invention that is used as a recording layer onto a support, astacked body for a holographic recording medium can be obtained. Thesupport may be disposed on one side of the recording layer formed fromthe cured product for a holographic recording medium or may be disposedon both sides.

The details of the recording layer and the support will be describedlater.

A method for forming the recording layer formed from the cured productfor a holographic recording medium is the same as that in the method forforming the holographic recording medium of the present invention.

[Holographic Recording Medium]

By subjecting the holographic recording medium composition of thepresent invention to interference exposure, the holographic recordingmedium of the present invention can be obtained.

A preferred mode of the holographic recording medium of the presentinvention will be described below.

The holographic recording medium of the present invention includes therecording layer and optionally includes the support and additionallayers. Generally, the holographic recording medium includes thesupport, and the recording layer and the additional layers are stackedon the support to form the holographic recording medium. However, whenthe recording layer and the additional layers have the strength anddurability required for the medium, the holographic recording medium mayinclude no support. Examples of the additional layers include aprotective layer, a reflecting layer, and an anti-reflection layer(anti-reflection film).

Preferably, the recording layer of the holographic recording medium ofthe present invention is formed from the holographic recording mediumcomposition of the present invention.

<Recording Layer>

The recording layer of the holographic recording medium of the presentinvention is a layer formed from the holographic recording mediumcomposition of the present invention, and information is recorded in therecording layer. The information is generally recorded as a hologram. Asdescribed later in detail in a recording method section, thepolymerizable monomer contained in the recording layer partiallyundergoes a chemical change such as polymerization during holographicrecording etc. Therefore, in the holographic recording medium afterrecording, part of the polymerizable monomer is consumed and present asa reacted compound such as a polymer.

No particular limitation is imposed on the thickness of the recordinglayer, and the thickness may be appropriately set in consideration ofthe recording method etc. The thickness is generally 1 μm or more andpreferably 10 μm or more and is generally 3000 μm or less and preferably2000 μm or less. When the thickness of the recording layer is equal toor more than the above lower limit, selectivity for holograms whenmultiple recording is performed on the holographic recording medium ishigh, and therefore the degree of multiple recording can be increased.When the thickness of the recording layer is equal to or less than theabove upper limit, the recording layer as a whole can be formeduniformly. Therefore, the holograms can have uniform diffractionefficiency, and multiple recording can be performed with a high S/Nratio.

Preferably, the rate of shrinkage of the recording layer due to exposureto light during information recording or reproduction is 0.5% or less.

<Support>

No particular limitation is imposed on the details of the support solong as it has the strength and durability required for the medium, andany support can be used.

No limitation is imposed on the shape of the support, and the support isgenerally formed into a flat plate or a film.

No limitation is imposed on the material forming the support, and thematerial may be transparent or may be opaque.

Examples of the transparent material for the support include: organicmaterials such as acrylic, polyethylene terephthalate, polyethylenenaphthoate, polycarbonate, polyethylene, polypropylene, amorphouspolyolefin, polystyrene, and cellulose acetate; and inorganic materialssuch as glass, silicon, and quartz. Of these, polycarbonate, acrylic,polyester, amorphous polyolefin, glass, etc. are preferred, andpolycarbonate, acrylic, amorphous polyolefin, and glass are morepreferred.

Examples of the opaque material for the support include: metals such asaluminum; and a coating prepared by coating the transparent support witha metal such as gold, silver, or aluminum or a dielectric such asmagnesium fluoride or zirconium oxide.

No particular limitation is imposed on the thickness of the support.Preferably, the thickness is in the range of generally 0.05 mm or moreand 1 mm or less. When the thickness of the support is equal to or morethan the above lower limit, the mechanical strength of the holographicrecording medium can be ensured, and warpage of the substrate can beprevented. When the thickness of the support is equal to or less thanthe above upper limit, the transmission amount of light can be ensured,and an increase in cost can be prevented.

The surface of the support may be subjected to surface treatment. Thesurface treatment is generally performed in order to improve theadhesion between the support and the recording layer. Examples of thesurface treatment include corona discharge treatment performed on thesupport and the formation of an undercoat layer on the support inadvance. Examples of the composition for the undercoat layer includehalogenated phenols, partially hydrolyzed vinyl chloride-vinyl acetatecopolymers, and polyurethane resins.

The surface treatment on the support may be performed for a purposeother than the improvement in adhesion.

Examples of such surface treatment include: reflecting coating treatmentin which a reflecting coating layer is formed using a metal materialsuch as gold, silver, or aluminum; and dielectric coating treatment inwhich a dielectric layer formed of magnesium fluoride, zirconium oxide,etc. is formed. Such a layer may be formed as a single layer, or two ormore layers may be formed.

The surface treatment on the support may be performed for the purpose ofcontrolling the gas and water permeability of the substrate. Forexample, when the support supporting the recording layer has thefunction of preventing permeation of gas and water, the reliability ofthe medium can be further improved.

The support may be disposed only on one of the upper and lower sides ofthe recording layer of the holographic recording medium of the presentinvention or may be disposed on both sides. When supports are disposedon both the upper and lower sides of the recording layer, at least oneof the supports is made transparent so that it can transmit activeenergy rays (such as excitation light, reference light, and reproductionlight).

When the holographic recording medium has the support on one side orboth sides of the recording layer, a transmission hologram or areflection hologram can be recorded. When a support having reflectioncharacteristics is used on one side of the recording layer, a reflectionhologram can be recorded.

A pattern for data addressing may be provided on the support. In thiscase, no limitation is imposed on the patterning method. For example,irregularities may be formed on the support itself, or the pattern maybe formed on the reflecting layer described later. The pattern may beformed using a combination of these methods.

<Protective Layer>

The protective layer is a layer for preventing a reduction in thesensitivity of the recording layer and deterioration in its storagestability due to oxygen or moisture. No limitation is imposed on thespecific structure of the protective layer, and any known protectivelayer can be used. For example, a layer formed of a water-solublepolymer, an organic or inorganic material, etc. can be formed as theprotective layer.

No particular limitation is imposed on the formation position of theprotective layer. The protective layer may be formed, for example, onthe surface of the recording layer or between the recording layer andthe support or may be formed on the outer surface side of the support.The protective layer may be formed between the support and anotherlayer.

<Reflecting Layer>

The reflecting layer is formed when the holographic recording mediumformed is of the reflection type. In the reflection holographicrecording medium, the reflecting layer may be formed between the supportand the recording layer or may be formed on the outer side of thesupport. Generally, it is preferable that the reflecting layer ispresent between the support and the recording layer.

Any known reflecting layer may be used, and a thin metal film, forexample, may be used.

<Anti-Reflection Film>

In each of the transmission and reflection holographic recordingmediums, an anti-reflection film may be disposed on the side on/fromwhich object light and reading light are incident/emitted or between therecording layer and the support. The anti-reflection film improves theefficiency of utilization of light and prevents the occurrence of aghost image.

Any known anti-reflection film may be used.

<Method for Producing Holographic Recording Medium>

No limitation is imposed on the method for producing the holographicrecording medium of the present invention. For example, the holographicrecording medium can be produced by coating the support with theholographic recording medium composition of the present inventionwithout using a solvent to form the recording layer. Any known coatingmethod can be used. Specific examples of the coating method include aspray method, a spin coating method, a wire bar method, a dippingmethod, an air knife coating method, a roll coating method, a bladecoating method, and a doctor roll coating method.

When a recording layer with a large thickness is formed, a method inwhich the holographic recording medium composition of the presentinvention is molded using a die, a method in which the holographicrecording medium composition is applied to a release film and punchedwith a die, etc. may be used.

The holographic recording medium may be produced by mixing theholographic recording medium composition of the present invention with asolvent or an additive to prepare a coating solution, coating thesupport with the coating solution, and then drying the coating solutionto form the recording layer. In this case also, any coating method canbe used. For example, any of the above described methods can be used.

No limitation is imposed on the solvent used for the coating solution.It is generally preferable to use a solvent that can dissolve thecomponent used sufficiently, provides good coating properties, and doesnot damage the support such as a resin substrate. Examples of thesolvent include: ketone-based solvents such as acetone and methyl ethylketone; aromatic solvents such as toluene and xylene; alcohol-basedsolvents such as methanol and ethanol; ketone alcohol-based solventssuch as diacetone alcohol; ether-based solvents such as tetrahydrofuran;halogen-based solvents such as dichloromethane and chloroform;cellosolve-based solvents such as methyl cellosolve and ethylcellosolve; propylene glycol-based solvents such as propylene glycolmonomethyl ether and propylene glycol monoethyl ether; ester-basedsolvents such as ethyl acetate and methyl 3-methoxypropionate;perfluoroalkyl alcohol-based solvents such as tetrafluoropropanol;highly polar solvents such as dimethylformamide and dimethyl sulfoxide;chain hydrocarbon-based solvents such as n-hexane; cyclichydrocarbon-based solvents such as cyclohexane and cyclooctane; andmixtures of these solvents.

One of these solvents may be used alone, or any combination of two ormore of them may be used at any ratio.

No limitation is imposed on the amount of the solvent used. However, interms of coating efficiency and handleability, it is preferable toprepare a coating solution with a solid concentration of about 1 toabout 1000% by weight.

When the resin matrix formed from components (a) and (b) in theholographic recording medium composition of the present invention isthermoplastic, the recording layer can be formed by molding theholographic recording medium composition of the present invention using,for example, an injection molding method, a sheet molding method, or ahot press method.

When the amount of volatile components in the resin matrix formed fromcomponents (a) and (b) is small and the resin matrix is photocurable orthermosetting, the recording layer can be formed by molding theholographic recording medium composition of the present invention using,for example, a reaction injection molding method or a liquid injectionmolding method. In this case, the molded product itself can be used as aholographic recording medium when the molded product has sufficientthickness, sufficient stiffness, sufficient strength, etc.

Examples of the holographic recording medium production method include:a production method in which the recording layer is formed by coatingthe support with the holographic recording medium composition fused byheat and cooling the holographic recording medium composition tosolidify the composition; a production method in which the recordinglayer is formed by coating the support with the holographic recordingmedium composition in liquid form and subjecting the holographicrecording medium composition to thermal polymerization to cure thecomposition; and a production method in which the recording layer isformed by coating the support with the holographic recording mediumcomposition in liquid form and subjecting the holographic recordingmedium composition to photopolymerization to cure the composition.

The thus-produced holographic recording medium can be in the form aself-supporting slab or disk and can be used for three-dimensional imagedisplay devices, diffraction optical elements, large-capacity memories,etc.

In particular, the holographic recording medium of the present inventionthat uses the holographic recording medium composition of the presentinvention has high M/#, is prevented from undergoing coloration, and istherefore useful for light guide plates for AR glasses.

<Large-Capacity Memory Applications>

Information is written (recorded)/read (reproduced) in/from theholographic recording medium of the present invention by irradiating themedium with light.

To record information, light capable of causing a chemical change of thepolymerizable monomer, i.e., its polymerization and a change inconcentration, is used as the object light (referred to also asrecording light).

For example, when information is recorded as a volume hologram, theobject light together with the reference light is applied to therecording layer to allow the object light to interfere with thereference light in the recording layer. In this case, the interferinglight causes polymerization of the polymerizable monomer and a change inits concentration within the recording layer. Therefore, theinterference fringes cause refractive index differences within therecording layer, and the information is recorded as a hologram throughthe interference fringes recorded in the recording layer.

To reproduce the volume hologram recorded in the recording layer,prescribed reproduction light (generally the reference light) is appliedto the recording layer. The applied reproduction light is diffracted bythe interference fringes. Since the diffracted light contains the sameinformation as that in the recording layer, the information recorded inthe recording layer can be reproduced by reading the diffracted lightusing appropriate detection means.

The wavelength ranges of the object light, the reproduction light, andthe reference light can be freely set according to their applicationsand may be the visible range or the ultraviolet range. Preferredexamples of such light include light from lasers with excellentmonochromaticity and directivity such as: solid lasers such as ruby,glass, Nd-YAG, Nd—YVO₄ lasers; diode lasers such as GaAs, InGaAs, andGaN lasers; gas lasers such as helium-neon, argon, krypton, excimer, andCO₂ lasers; and dye lasers including dyes.

No limitation is imposed on the amounts of irradiation with the objectlight, the reproduction light, and the reference light, and theseamounts can be freely set so long as recording and reproduction arepossible. If the amounts of irradiation are extremely small, thechemical change of the polymerizable monomer is too incomplete, and theheat resistance and mechanical properties of the recording layer may notbe fully obtained. If the amounts of irradiation are extremely large,the components of the recording layer (the components of the holographicrecording medium composition of the present invention) may deteriorate.Therefore, the object light, the reproduction light, and the referencelight are applied at generally 0.1 J/cm² or more and 20 J/cm² or lessaccording to the chemical composition of the holographic recordingmedium composition of the present invention used to form the recordinglayer, the type of photopolymerization initiator, the amount of thephotopolymerization initiator mixed, etc.

Examples of the holographic recording method include a polarizedcollinear holographic recording method and a reference light incidenceangle multiplexing holographic recording method. When the holographicrecording medium of the present invention is used as a recording medium,good recording quality can be provided using any of the recordingmethods.

The holographic recording medium of the present invention ischaracterized in that M/# and light transmittance are high and that theamount of change in chromaticity is small.

The refractive index modulation generated by holographic recording ismeasured as diffraction efficiency. The holographic recording can beperformed in a multiplexed manner. Examples of the multiplexing methodinclude an angle multiplexing method in which recording is performedwhile incident angles of light beams intersecting at a fixed angle arechanged, a shift multiplexing method in which the recording position ischanged while the incident angles are not changed, and a wavelengthmultiplexing method in which recording is performed while the wavelengthis changed. Of these, the angle multiplexing is simple and easy, and thematerials and the performance of each components can be specified.

The value M/#, which is the sum of the square roots of diffractionefficiency values over the entire multiplexing range, is a measure ofthe recording capacity. The larger the value M/#, the better theperformance of the medium. Generally, as the content of thepolymerizable monomer increases, the diffraction efficiency increases,and the M/# also increases.

The M/# of the recording layer of the holographic recording medium ofthe present invention is 20 or more, preferably 30 or more, and stillmore preferably 35 or more, when the thickness of the recording layerused for the evaluation is 500 μm. As described above, the value of theM/# is preferably as large as possible, and the upper limit of the M/#can vary according to the necessary recording capacity.

The M/# is measured by a method described later in Examples.

Layers other than the recording layer such as the substrate, theprotective layer, and the reflecting layer, etc. included in theholographic recording medium have no significant influence on the M/#value. Therefore, the M/# values of mediums having different layerstructures excluding the recording layer can be directly compared witheach other.

The light transmittance of the holographic recording medium of thepresent invention is an important indicator. The transmittance beforerecording is determined by the reflectance at the substrate-airinterface and absorption by the unreacted initiator in the recordingmedium and can therefore be computed using the reflectance at thesubstrate-air interface, the molar absorption coefficient andconcentration of the initiator in the holographic recording medium, andthe thickness of the holographic recording medium. The lighttransmittance obtained by computation is referred to as designtransmittance.

The design transmittance of the holographic recording medium is computedfrom the following formula using the reflectance of the substrate-airinterface, the molar absorption coefficient of the photopolymerizationinitiator in the holographic recording medium, the concentration of thephotopolymerization initiator in the holographic recording medium, andthe thickness of the holographic recording medium.T=R ²×10^((−ε×c×d))

Here, T is the design transmittance, and R is the reflectance at thesubstrate-air interface. ε is the molar absorption coefficient of thephotopolymerization initiator, and c is the concentration of thepolymerization initiator in the holographic recording medium. d is thethickness of the holographic recording medium.

In some cases, the transmittance measured before recording may be lowerthan the design transmittance. This suggests that the holographicrecording medium is tinted for some other reasons. The coloration of theholographic recording medium leads to a reduction in image quality whenthe holographic recording medium is used for light guide plates for ARglasses. Therefore, it can be said that the smaller the differencebetween the design transmittance and the transmittance before recording,the higher the performance of the holographic recording medium.

Preferably, the light used to measure the light transmittance has awavelength equal to or close to the recording wavelength. However,before recording, the chemical change of the photopolymerizationinitiator in the recording layer included in the holographic recordingmedium is significant, and the light transmittance varies with time.Therefore, the light transmittance before recording must be measured ina sufficiently short time (about 1 second) in order to obtain a reliableand reproduceable measurement value. However, after recording, thephotopolymerization initiator has been consumed, and no temporal changeoccurs, and therefore the light transmittance can be measured withoutconcern for the measurement time.

It is generally preferable that the design transmittance coincides withthe transmittance before recording. When they differ from each other,the difference is preferably about 3% or less, more preferably 2% orless, and still more preferably 1% or less. In the holographic recordingmedium of the present invention, when the recording layer has athickness of 500 μm, its transmittance evaluated before recording can bewithin the above range and can be generally 3% or less and preferably 1%or less. Specifically, the light transmittance is measured using amethod described later in Examples.

<AR Glass Light Guide Plate Applications>

Volume holograms are recorded in the holographic recording medium of thepresent invention in the same manner as in the large-capacity memoryapplications.

The volume holograms recorded in the recording layer are irradiated withprescribed reproduction light through the recording layer. The appliedreproduction light is diffracted by the interference fringes. In thiscase, even when the wavelength of the reproduction light does notcoincide with the wavelength of the recording light, diffraction occurswhen the Bragg condition for the interference fringes is satisfied.Therefore, by recording interference fringes corresponding to thewavelengths and incident angles of reproduction light beams to bediffracted, the reproduction light beams in a wide wavelength range canbe diffracted, and the color display range of AR glasses can beincreased.

By recording interference fringes corresponding to the wavelength anddiffraction angle of reproduction light, the reproduction light enteringfrom the outside of the holographic recording medium can be guided tothe inside of the holographic recording medium, and the reproductionlight guided inside the holographic recording medium can be reflected,split, and expanded or reduced in size. Moreover, the reproduction lightguided inside the holographic recording medium can be emitted to theoutside of the holographic recording medium. This allows the viewingangle of AR glasses to be increased.

The wavelength ranges of the object light and the reproduction light canbe freely set according to their applications, and the object light andthe reproduction light may be in the visible range or in the ultravioletrange. Preferred examples of the object light and the reproduction lightinclude light from the above-described lasers. The reproduction light isnot limited to light from a laser etc., and display devices such asliquid crystal displays (LCDs), organic electroluminescent displays(OLEDs), etc. can also be used preferably.

No limitation is imposed on the amounts of irradiation with the objectlight, the reproduction light, and the reference light, and theseamounts can be freely set so long as recording and reproduction arepossible. If the amounts of irradiation are extremely small, thechemical change of the polymerizable monomer is too incomplete, and theheat resistance and mechanical properties of the recording layer may notbe fully obtained. If the amounts of irradiation are extremely large,the components of the recording layer (the components of the holographicrecording medium composition of the present invention) may deteriorate.Therefore, the object light, the reproduction light, and the referencelight are applied at generally 0.1 J/cm² or more and 20 J/cm² or lessaccording to the chemical composition of the holographic recordingmedium composition of the present invention used to form the recordinglayer, the type of photopolymerization initiator, the amount of thephotopolymerization initiator mixed, etc.

EXAMPLES

The present invention will be described in more detail by way ofExamples. However, the present invention is not limited to the Examplesso long as the invention does not depart from the scope thereof.

[Materials Used]

Raw materials of compositions used in the Examples, and ComparativeExamples are as follows.

Component (a): isocyanate group-containing compound—DURANATE (registeredtrademark) TSS-100: hexamethylene diisocyanate-based polyisocyanate (NCO17.6%) (manufactured by Asahi Kasei Corporation)

Component (b): isocyanate-reactive functional group-containing compound

-   -   PLACCEL PCL-205U: polycaprolactonediol (molecular weight 530)        (manufactured by Daicel Corporation)    -   PLACCEL PCL-305: polycaprolactonetriol (molecular weight 550)        (manufactured by Daicel Corporation)

Component (c): polymerizable monomer

-   -   HLM101:        2,2-bis(4-dibenzothiophenylthiomethyl)-3-(4-dibenzothiophenylthio)propyl        acrylate    -   BDTPA: 2,4-Bis(4-dibenzothiophenyl)-1-phenyl acrylate

Component (d): photopolymerization initiator

-   -   HLI02:        1-(9-ethyl-6-cyclohexanoyl-9H-carbazol-3-yl)-1-(O-acetyloxime)glutaric        acid methyl ester

Component (e): a compound having an isocyanate group or anisocyanate-reactive functional group and further having a nitroxylradical group

-   -   3-HO-ABNO: 3-hydroxy-9-azabicyclo[3.3.1]nonane N-oxyl    -   5-HO-AZADO: 5-hydroxy-2-azatricyclo[3.3.1.1^(3,7)]decane N-oxyl

Component (f): curing catalyst

-   -   Octylic acid solution of tris(2-ethylhexanoate)bismuth (amount        of effective component 56% by weight)

Component (g): additional component

-   -   Methyl linoleate (LM): manufactured by TOKYO CHEMICAL INDUSTRY        Co., Ltd.    -   TEMPOL: 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl free        radical (manufactured by TOKYO CHEMICAL INDUSTRY Co., Ltd.)

Methyl linoleate was used instead of component (e) in ComparativeExample 1.

TEMPOL was used instead of component (e) in Comparative Examples 2 to 9.

Hereinafter, component (e) and component (g) may each be referred to asan “additive.”

Example 1

<Preparation of 3-HO-ABNO Master Batch>

0.15 g of 3-HO-ABNO was dissolved in 2.85 g of DURANATE™ TSS-100. Next,0.002 g of an octylic acid solution of tris(2-ethylhexanoate)bismuth wasdissolved in the above mixture, and the resulting mixture was stirred at45° C. under reduced pressure to allow the mixture to react for 2 hours.FT-IR was used to observe the disappearance of a peak at 3450 cm⁻¹originating from a hydroxy group in 3-HO-ABNO to thereby check thereaction of the hydroxy group in the 3-HO-ABNO to an isocyanate group.12 g of DURANATE™ TSS-100 was further added to thereby produce a3-HO-ABNO master batch.

<Preparation of Holographic Recording Medium Composition>

0.362 g of a polymerizable monomer HLM101 and 0.0145 g of aphotopolymerization initiator HLI02 were dissolved in 3.69 g ofDURANATE™ TSS-100 to obtain solution A.

Separately, 2.42 g of PLACCEL PCL-205U (polycaprolactonediol (molecularweight 530)) and 1.04 g of PLACCEL PCL-305 (polycaprolactonetriol(molecular weight 550)) were mixed (PLACCEL PCL-205U:PLACCELPCL-305=70:30 (weight ratio)), and 0.0003 g of an octylic acid solutionof tris(2-ethylhexanoate)bismuth was dissolved therein to obtainsolution B.

Each of solutions A and B was degassed at 45° C. under reduced pressurefor 2 hours. Then 2.36 g of solution A, 2.35 g of solution B, and 0.285g of the 3-HO-ABNO master batch were mixed under stirring and degassedin a vacuum for several minutes.

Then the vacuum degassed solution mixture was poured onto a microscopeslide with 0.5 mm-thick spacer sheets placed on two opposite edges, andanother microscope slide was placed thereon. Clips were used to fix theedges, and heating was performed at 80° C. for 24 hours to produce aholographic recording medium composition evaluation sample. In thisevaluation sample, a recording layer with a thickness of 0.5 mm wasformed between the microscope slides used as covers.

In this holographic recording medium composition, the content of thepolymerizable monomer denoted as component (c) was 4.6% by weight, andthe ratio of the photopolymerization initiator denoted as component (d)to the polymerizable monomer denoted as component (c) was 4% by weight.The molar ratio of 3-HO-ABNO to the photopolymerization initiatordenoted as component (d) was 1, and the ratio (OH/NCO) of hydroxy groupsin component (b) to isocyanate groups in component (a) was 1.0.

Examples 2 to 4

Holographic recording medium compositions and holographic recordingmedium composition evaluation samples were produced in the same manneras in Example 1 except that the 3-HO-ABNO master batch was added suchthat the molar ratio of the 3-HO-ABNO to the photopolymerizationinitiator denoted as component (d) was equal to a value shown in Table1.

Examples 5 to 8

Holographic recording medium compositions and holographic recordingmedium composition evaluation samples were produced in the same manneras in Example 1 except that 5-HO-AZADO was used instead of 3-HO-ABNO andthat the 5-HO-AZADO master batch was added such that the molar ratio ofthe 5-HO-AZADO to the photopolymerization initiator denoted as component(d) was equal to a value shown in Table 1.

Examples 9 to 10

Holographic recording medium compositions and holographic recordingmedium composition evaluation samples were produced in the same manneras in Example 5 except that BDTPA was used instead of HLM101, that themixing ratio of PLACCEL PCL-205U to PLACCEL PCL-305 was changed toPLACCEL PCL-205U:PLACCEL PCL-305=90:10 (weight ratio), and that theamount and molar ratio of each of the components added are equal tovalues shown in Table 1.

Comparative Example 1

<Preparation of Holographic Recording Medium Composition>

0.386 g of a polymerizable monomer HLM101, 0.0154 g of aphotopolymerization initiator HLI02, and 3.78 mg of methyl linoleatewere dissolved in 4.04 g of DURANATE™ TSS-100 to obtain solution A.

Separately, 2.25 g of PLACCEL PCL-205U (polycaprolactonediol (molecularweight 530)) and 0.964 g of PLACCEL PCL-305 were mixed(polycaprolactonetriol (molecular weight 550)), and 0.0003 g of anoctylic acid solution of tris(2-ethylhexanoate)bismuth was dissolvedtherein to obtain solution B.

Each of solutions A and B was degassed at 45° C. under reduced pressurefor 2 hours. Then 2.65 g of solution A and 2.35 g of solution B weremixed under stirring and degassed in a vacuum for several minutes.

Then the vacuum degassed solution mixture was poured onto a microscopeslide with 0.5 mm-thick spacer sheets placed on two opposite edges, andanother microscope slide was placed thereon. Clips were used to fix theedges, and heating was performed at 80° C. for 24 hours to produce aholographic recording medium composition evaluation sample. In thisevaluation sample, a recording layer with a thickness of 0.5 mm wasformed between the microscope slides used as covers.

In this holographic recording medium composition, the content of thepolymerizable monomer denoted as component (c) was 4.6% by weight, andthe ratio of the photopolymerization initiator denoted as component (d)to the polymerizable monomer denoted as component (c) was 4% by weight.The molar ratio of methyl linoleate to the photopolymerization initiatorwas 0.42.

Comparative Examples 2 to 6

Holographic recording medium compositions and holographic recordingmedium composition evaluation samples were produced in the same manneras in Example 1 except that TEMPOL was used instead of 3-HO-ABNO andthat a TEMPOL master batch was added such that the molar ratio of theTEMPOL to the photopolymerization initiator denoted as component (d) wasequal to a value shown in Table 2.

Comparative Examples 7 to 9

Holographic recording medium compositions and holographic recordingmedium composition evaluation samples were produced in the same manneras in Comparative Example 2 except that BDTPA was used instead ofHLM101, that the mixing ratio of PLACCEL PCL-205U and PLACCEL PCL-305was changed to PLACCEL PCL-205U:PLACCEL PCL-305=90:10 (weight ratio),and that the amount and molar ratio of each of the components used wereequal to values shown in Table 2.

[Holographic Recording and Evaluation Methods]

Holographic recording medium composition evaluation samples produced inExamples and Comparative Examples were used to perform holographicrecording and evaluate the holographic recording performance of eachholographic recording medium using procedures described below.

Holographic recording was performed using a semiconductor laser with awavelength of 405 nm. An exposure device shown in FIG. 1 was used toperform two-beam plane-wave holographic recording at an exposure powerdensity per beam of 7.5 mW/cm².

Multiple recording was performed 61 times in the same position of themedium in steps of 0.6° in the range of from −18° to 18°, and the sum ofthe square root of diffraction efficiency values was used as M/# (Mnumber).

Details will next be described.

(Holographic Recording)

FIG. 1 is a structural diagram showing the outline of the device usedfor holographic recording.

In FIG. 1 , S represents a holographic recording medium sample, and M1to M3 represent mirrors. PBS represents a polarizing beam splitter. L1represents a recording laser light source emitting light with awavelength of 405 nm (a single mode laser (“L1” in FIG. 1 ) manufacturedby TOPTICA Photonics and capable of emitting light with a wavelength ofabout 405 nm), and L2 represents a reproduction laser light sourceemitting light with a wavelength of 633 nm. PD1, PD2, and PD3 representphotodetectors. 1 represents an LED unit.

As shown in FIG. 1 , a light beam with a wavelength of 405 nm was splitusing the polarizing beam splitter (“PBS” in the figure) into two beamsintersecting on a recording surface such that the angle therebetween was37.3°. In this case, the light beam was split such that a bisector ofthe angle between the two beams was perpendicular to the recordingsurface, and the two beams obtained by splitting the light beam wereapplied such that the vibration planes of the electric field vectors ofthe two beams were perpendicular to a plane including the twointersecting beams.

After the holographic recording, a He—Ne laser capable of emitting lightwith a wavelength of 633 nm (V05-LHP151 manufactured by Melles Griot:“L2” in the figure) was used, and the light was applied to the recordingsurface at an angle of 30.0°. The diffracted light was detected using aphoto diode and a photosensor amplifier (S2281 and C9329: manufacturedby Hamamatsu Photonics K.K., “PD1” in the figure) to determine whetheror not the holographic recording was correctly performed. Thediffraction efficiency of each hologram is given by the ratio of theintensity of the diffracted light to the sum of the intensity oftransmitted light and the intensity of the diffracted light.

(Measurement of M/#)

Multiple recording was performed 61 times while the sample was movedwith respect to the optical axis such that the angle between the opticalaxis and the sample (the angle between the normal to the sample and abisector of the interior angle of the two beams, i.e., incident lightbeams from the mirrors M1 and M2 in FIG. 1 , at the intersection of thebeams) was changed from −18° to 18° in steps of 0.6°.

After the multiple recording was performed 61 times, the LED unit (1 inthe figure, center wavelength 405 nm) was turned on for a given periodof time to completely consume the remaining initiator and the remainingmonomer. This process is referred to as postexposure. The power of theLED was 50 mW/cm², and the irradiation was performed such that theintegrated energy was 3 J/cm².

Next, light (wavelength 405 nm) from the mirror M1 in FIG. 1 wasapplied, and the diffraction efficiency was measured in the angle rangeof from −19° to 19°. The sum of the square roots of the obtaineddiffraction efficiency values over the entire multiple recording rangewas used as M/#.

A plurality of optical recording mediums prepared were used. Theevaluation was performed a plurality of times under differentirradiation energy conditions, i.e., while the irradiation energy at thebeginning of irradiation was increased or decreased and the totalirradiation energy was increased or decreased. A search for conditionsunder which the monomer contained was almost completely consumed by thetime the multiple recording was repeated 61 times (the M/# substantiallyreached equilibrium by the time the multiple recording was repeated 61times) while the diffraction efficiency in each recording operation wasmaintained at several percent or more was performed in order to maximizethe M/#. The maximum value obtained was used as the M/# of the medium.

(Computation of Sensitivity)

A value obtained by dividing a value obtained by multiplying the M/# by0.8 by the irradiation energy applied until the M/# reached 80% wasdefined as sensitivity.

(Measurement of Transmittance Before Recording and Transmittance afterRecording)

The transmittance before recording of an evaluation sample wasdetermined before recording by measuring the ratio of the power of thetransmission light to the power of the incident light.

Moreover, the transmittance after recording of the evaluation samplesubjected to postexposure was determined after recording by measuringthe ratio of the power of the transmission light to the power of theincident light.

The results of evaluation of the holographic recording mediums producedin Examples 1 to 10 and Comparative Examples 1 to 9 using theabove-described methods are shown in Tables 1 and 2 below.

The relation between M/# and the amount of an additive added (molarratio of the additive/component (d)) is shown in FIG. 2 .

In Tables 1 and 2, a numerical value in parentheses in the component (b)column indicates the ratio (% by weight) of PLACCEL PCL-205U to thetotal of PLACCEL PCL-205U and PLACCEL PCL-305.

A numerical value in parentheses in the component (c) column indicatesthe content (% by weight) of component (c) in the holographic recordingmedium composition.

A numerical value in parentheses in the component (d) column indicatesthe weight ratio of “component (d)/component (c)” in the holographicrecording medium composition.

“Total content of component (a)+(b)” indicates the total content ofcomponent (a) and component (b) in the holographic recording mediumcomposition.

In each of the Examples and Comparative Examples, the ratio (OH/NCO) ofhydroxy groups in component (b) to isocyanate groups in component (a)was 1.0.

In each of the Examples and Comparative Examples, 1.0 was obtained.

TABLE 1 Total content of component Additive Trans- Trans- (a) +Additive/ mittance mittance Component Component Component componentcomponent before after Component (b) (% by (c) (% by (d) (b) (% by (d)Sensitivity recording recording (a) weight) weight) ((d)/(c)) weight)Type molar ratio M/# (cm/mJ) (%) (%) Example TSS-100 PCL-205U/ HLM101HLI02 95.4 3-HO-ABNO   1/1 33.3 1.44 69 75  1 305 (70) (4.6) (4) ExampleTSS-100 PCL-205U/ HLM101 HLI02 95.4 3-HO-ABNO 1.43/1 39.3 1.59 69 77  2305 (70) (4.6) (4) Example TSS-100 PCL-205U/ HLM101 HLI02 95.4 3-HO-ABNO  2/1 41.7 1.27 69 79  3 305 (70) (4.6) (4) Example TSS-100 PCL-205U/HLM101 HLI02 95.4 3-HO-ABNO  2.5/1 37.2 1.19 69 76  4 305 (70) (4.6) (4)Example TSS-100 PCL-205U/ HLM101 HLI02 95.4 5-HO-AZADO 0.72/1 33.8 1.4469 79  5 305 (70) (4.6) (4) Example TSS-100 PCL-205U/ HLM101 HLI02 95.45-HO-AZADO   1/1 40.1 1.59 69 69  6 305 (70) (4.6) (4) Example TSS-100PCL-205U/ HLM101 HLI02 95.4 5-HO-AZADO 1.43/1 48.2 1.36 69 78  7 305(70) (4.6) (4) Example TSS-100 PCL-205U/ HLM101 HLI02 95.4 5-HO-AZADO1.75/1 34.6 1.01 69 79  8 305 (70) (4.6) (4) Example TSS-100 PCL-205U/BDTPA HLI02 97.0 5-HO-AZADO   1/1 33.7 1.31 69 88  9 305 (90) (3.0)(6.1) Example TSS-100 PCL-205U/ BDTPA HLI02 97.0 5-HO-AZADO 1.43/1 44.61.34 69 87 10 305 (90) (3.0) (6.1)

TABLE 2 Total content of component Additive Trans- Trans- (a) +Additive/ mittance mittance Component Component Component componentcomponent before after Component (b) (% by (c) (% by (d) (b) (% by (d)Sensitivity recording recording (a) weight) weight) ((d)/(c)) weight)Type molar ratio M/# (cm/mJ) (%) (%) Comparative TSS-100 PCL-205U/HLM101 HLI02 95.4 LM 0.42/1 20.7 0.78 69 82 Example 1 305 (70) (4.6) (4)Comparative TSS-100 PCL-205U/ HLM101 HLI02 95.4 TEMPOL 0.24/1 24.8 1.2069 80 Example 2 305 (70) (4.6) (4) Comparative TSS-100 PCL-205U/ HLM101HLI02 95.4 TEMPOL 0.72/1 34.7 1.41 69 74 Example 3 305 (70) (4.6) (4)Comparative TSS-100 PCL-205U/ HLM101 HLI02 95.4 TEMPOL   1/1 39.0 1.1869 82 Example 4 305 (70) (4.6) (4) Comparative TSS-100 PCL-205U/ HLM101HLI02 95.4 TEMPOL  1.2/1 35.0 1.04 69 85 Example 5 305 (70) (4.6) (4)Comparative TSS-100 PCL-205U/ HLM101 HLI02 95.4 TEMPOL 1.43/1 23.6 0.7069 86 Example 6 305 (70) (4.6) (4) Comparative TSS-100 PCL-205U/ BDTPAHLI02 97.0 TEMPOL 0.72/1 35.0 1.47 69 82 Example 7 305 (90) (3.0) (6.1)Comparative TSS-100 PCL-205U/ BDTPA HLI02 97.0 TEMPOL   1/1 32.6 1.24 6982 Example 8 305 (90) (3.0) (6.1) Comparative TSS-100 PCL-205U/ BDTPAHLI02 97.0 TEMPOL 1.43/1 17.0 0.55 69 85 Example 9 305 (90) (3.0) (6.1)

In the holographic recording mediums in the Examples and also in theholographic recording mediums in the Comparative Examples, the M/#varied according to the amount of the additive added.

In Comparative Examples 2 to 9, TEMPOL was used as the additive. WhenHLM101 was used as component (c), the M/# was highest when the molarratio of the amount of TEMPOL added to the amount of thephotopolymerization initiator denoted as component (d) was 1, and themaximum value was 39.0.

When BDTPA was used as component (c), the M/# was highest when the molarratio of the amount of TEMPOL added to the amount of thephotopolymerization initiator denoted as component (d) was 0.72, and themaximum value was 35.0.

In Examples 1 to 4, 3-HO-ABNO denoted as component (e) was used. The M/#was highest when the molar ratio of the amount of 3-HO-ABNO added to theamount of the photopolymerization initiator denoted as component (d) was2, and the maximum value was 41.7.

In Examples 5 to 10, 5-HO-AZADO denoted as component (e) was used. WhenHLM101 was used as component (c), the M/# was highest when the molarratio of the amount of 5-HO-AZADO added to the amount of thephotopolymerization initiator denoted as component (d) was 1.43, and themaximum value was 48.2.

When BDTPA was used as component (c), the M/# was highest when the molarratio of the amount of 5-HO-AZADO added to the amount of thephotopolymerization initiator denoted as component (d) was 1.43, and themaximum value was 44.6.

As described above, in the optical element applications such as lightguide plates for AR glasses, the higher the M/#, the brighter aprojected image, and the wider the viewing angle. An increase in M/# bya factor of 1.25 (39.0→48.2) corresponds to an increase in diffractionefficiency by a factor of 1.5 and an increase in brightness by a factorof 1.5.

In the memory applications, by improving the M/#, the recording capacitycan be improved.

Therefore, when the holographic recording medium is used for theseapplications, components (e) in the present invention used in theExamples are preferred to the TEMPOL used in the Comparative Examples.

In the TEMPOL used in Comparative Examples 2 to 9, the periphery of thenitroxyl radical group that traps a radical is sterically hindered byfour methyl groups.

However, in component (e) used in Examples 1 to 10, the periphery of thenitroxyl radical group is not sterically hindered. Therefore, theradical trapping efficiency of the nitroxyl radical group is high, andthis may be the reason for the improvement in M/#.

This feature may be commonly observed in compounds in which theperiphery of the nitroxyl radical group is not sterically hindered andcompounds with a low redox potential.

[Evaluation of Archival Life (Accelerated Test]]

Three recorded holographic recording mediums were prepared for each ofComparative Examples 4 and 7 and Examples 3, 7, and 10. Each holographicrecording medium was heated at 80° C. in dry air for a prescribed timeshown in Tables 4 to 8. After the heating, the M/# of a holographicrecording section of each medium was measured. The measurement of theM/# was performed three times, and the highest value was used as theM/#.

The diffraction efficiency (an irradiation angle of −19° to +19°) ofeach holographic recording medium was measured at the beginning of anaccelerated test and at the end of the accelerated test.

The chemical compositions of the holographic recording mediumcompositions in Comparative Examples 4 and 7 and

Examples 3, 7, and 10 are shown in Table 3, and the results ofmeasurement of M/# are shown in Tables 4 to 8.

The results of evaluation in Comparative Examples 4 and 7 and Examples3, 7, and 10 are shown in: FIGS. 3A, 3B, and 3C; FIGS. 4A, 4B, and 4C;FIGS. 5A, 5B, and 5C; FIGS. 6A, 6B, and 6C; and FIGS. 7A, 7B, and 7C,respectively.

TABLE 3 Component (b) Content of Component (d)/ Additive/ polyolcomponent (c) component (c) component (d) (% by weight) (% by weight) (%by weight) (molar ratio) Comparative PCL-205U (70) HLM101 HLI02 TEMPOLExample 4  PCL-305 (30)   (4.6) (4) (1) Comparative PCL-205U (90) BDTPAHLI02 TEMPOL Example 7  PCL-305 (10)   (3.0) (6.1) (0.72) Example 3 PCL-205U (70) HLM101 HLI02 3-HO-ABNO PCL-305 (30)   (4.6) (4) (2)Example 7  PCL-205U (70) HLM101 HLI02 5-HO-AZADO PCL-305 (30)   (4.6)(4) (1.43) Example 10 PCL-205U (90) BDTPA HLI02 5-HO-AZADO PCL-305(10)   (3.0) (6.1) (1.43)

Comparative Example 4

TABLE 4 M/# Additive Acceleration time TEMPOL 0 hr 240 hr 400 hr 800 hrNo. 1 39.6 35.8 33.1 28.3 No. 2 39.1 36.5 34 28.1 No. 3 138.9 36.3 34.129.8

Comparative Example 7

TABLE 5 M/# Additive Acceleration time TEMPOL 0 hr 230 hr 398 hr 870 hrNo. 1 35   33.6 32.6 29.2 No. 2 34.5 32.6 32.1 27.9 No. 3 35.2 33.7 32.328.6

Example 3

TABLE 6 M/# Additive Acceleration time 3-HO-ABNO 0 hr 240 hr 400 hr 800hr No. 1 41.6 41   40.4 39.7 No. 2 40.7 40.5 39.6 39.2 No. 3 40.7 40.439.8 39.2

Example 7

TABLE 7 M/# Additive Acceleration time 5-OH-AZADO 0 hr 240 hr 408 hr 857hr No. 1 50.1 49.4 49.3 49   No. 2 48.7 48.3 48.2 47.7 No. 3 51.4 50  50.3 50.2

Example 10

TABLE 8 M/# Additive Acceleration time 5-HO-AZADO 0 hr 240 hr 400 hr 800hr No. 1 44.5 44.1 44.2 44.1 No. 2 44.1 43.4 43.2 43   No. 3 43   41.842.2 42  

In Comparative Example 7, the M/# decreased with acceleration time, andthe decrease in M/# was 15 to 20% at 870 hr. As shown in FIGS. 3A, 3B,and 3C, this corresponds to a reduction in diffraction efficiency toabout ⅔. In Comparative Example 4 also, the M/# and the diffractionefficiency decreased significantly.

Unlike smartphones, AR glasses, which are one type of wearable device,are assumed to be always worn. Therefore, the AR glasses are more oftenexposed to hot environments such as the inside of an automobile insummer, a high temperature area of a plant, etc. It is thereforeimportant that, when the holographic recording medium is used for theoptical element applications such as light guide plates for AR glasses,the diffraction efficiency does not change over a long period of timeeven when the holographic recording medium is exposed to a hotenvironment.

In Comparative Examples 4 and 7 in which the additive TEMPOL was used,the M/# decreased with time in the accelerated test at 80° C. Therefore,if a holographic recording medium using TEMPOL were used for the opticalelement applications such as light guide plates for AR glasses, thebrightness of a projected image would be reduced with time, and theviewing angle would be narrowed.

In Examples 3, 7, and 10, almost no reduction in M/# with theacceleration time was observed. For example, in Example 7, the reductionin M/# was about 2% at 857 hr. Therefore, even when these holographicrecording mediums are used for the optical element applications such aslight guide plates for AR glasses, a reduction in brightness of aprojected image, deterioration in color unevenness, and a reduction inviewing angle will not occur during use in a high-temperatureenvironment.

In the memory applications, even after the recorded medium is exposed toa high-temperature environment for a long time, information will not belost.

Therefore, when the holographic recording medium is used for theseapplications, the additives used in the Examples are preferred to theadditives in Comparative Examples.

In the additive TEMPOL used in Comparative Examples 4 and 7, four methylgroups are present in the periphery of the nitroxyl radical group.Therefore, the potential energy of the NO—C bond in the dormant speciesis high due to steric hindrance by the methyl groups, so that theactivation energy of thermal bond cleavage is small, i.e., thermalcleavage easily occurs.

In the additive 5-HO-AZADO and 3-HO-ABNO used in Examples 3, 7, and 10,the periphery of the nitroxyl radical group is not sterically hindered.Therefore, the potential energy of the NO—C bond of the dormant speciesis low, and the activation energy of thermal bond cleavage is large,i.e., thermal cleavage is unlikely to occur.

This feature may be commonly observed in compounds in which theperiphery of the nitroxyl radical group is not sterically hindered andcompounds with a low redox potential.

Although the present invention has been described in detail by way ofthe specific modes, it is apparent for those skilled in the art thatvarious changes can be made without departing from the spirit and scopeof the present invention.

The present application is based on Japanese Patent Application No.2018-150115 filed on Aug. 9, 2018, the entire contents of which areincorporated herein by reference.

REFERENCE SIGNS LIST

-   -   S holographic recording medium    -   M1, M2, M3 mirror    -   L1 semiconductor laser light source for recording light    -   L2 laser light source for reproduction light    -   PD1, PD2, PD3 photodetector    -   PBS polarizing beam splitter    -   1 LED unit

The invention claimed is:
 1. A holographic recording medium composition, comprising: a matrix resin; component (c): a polymerizable monomer; component (d): a photopolymerization initiator; component (e): a compound having an isocyanate group or an isocyanate-reactive functional group and further having a nitroxyl radical group, wherein component (e) comprises component (e-2) below: component (e-2): a compound represented by formula (1) below:

wherein, in formula (I), C₁ and C₂ each represent a carbon atom; two R^(X)s in formula (I) are each the same as R^(A) or are bonded together and represent a direct bond or a linking group, the direct bond or the linking group covalently bridging the two carbon atoms C₁ and C₂; and, when R^(X) and R^(X) represent the linking group, the linking group is represented by —C(—R^(A))₂— or —C(—R^(A))₂—C(—R^(A))₂—, wherein, in formula (I) and the linking group, each R^(A) represents a substituent selected from a hydrogen atom, halogen atoms, C1-12 alkyl groups, a hydroxy group, hydroxy(C1-12 alkyl) groups, an amino group, amino(C1-12 alkyl) groups, an isocyanate group, and isocyanate(C1-12 alkyl) groups; in formula (I) and the linking group, one or two R^(A)s are bonded to each carbon atom in a bicyclic ring structure or a tricyclic ring structure; when the number of substituents on a carbon atom is two or more, the substituents may be the same or different; and, in formula (I) and the linking group, the plurality of R^(A)s may be the same or different, provided that at least one of R^(A)s in formula (I) and the linking group is at least one substituent selected from a hydroxy group, hydroxy(C1-12 alkyl) groups, an amino group, amino(C1-12 alkyl) groups, and an isocyanate group, and wherein a molar ratio of component (e-2) to component (d) (component (e-2)/component (d)) in the composition is 1.43 or more and 10 or less.
 2. The holographic recording medium composition according to claim 1, wherein the matrix resin is obtained by reacting component (a): an isocyanate group-containing compound and component (b): an isocyanate-reactive functional group-containing compound.
 3. The holographic recording medium composition according to claim 2, wherein a ratio of a total weight of a propylene glycol unit and a tetramethylene glycol unit included in component (b) to a total weight of component (a) and component (b) is 30% or less.
 4. The holographic recording medium composition according to claim 3, wherein a ratio of the weight of a caprolactone unit contained in component (b) to the total weight of component (a) and component (b) is 20% or more.
 5. The holographic recording medium composition according to claim 2, further comprising component (f) below: component (f): a curing catalyst.
 6. The holographic recording medium composition according to claim 2, wherein a total content of component (a) and component (b) in the composition is 0.1% by weight or more and 99.9% by weight or less, and wherein a ratio of a number of isocyanate-reactive functional groups contained in component (b) to a number of isocyanate groups contained in component (a) is 0.1 or more and 10.0 or less.
 7. The holographic recording medium composition according to claim 1, wherein the compound represented by formula (I) is a compound represented by one of formulas (Ia) to (Ic) below:

wherein, in formulas (Ia) to (Ic), each R^(A) is the same as that in formula (I).
 8. The holographic recording medium composition according to claim 1, wherein a content of component (c) in the composition is 0.1% by weight or more and 80% by weight or less, and wherein a ratio of an amount of component (d) to an amount of component (c) is 0.1% by weight or more and 20% by weight or less.
 9. A cured product obtained by a process comprising curing the holographic recording medium composition according to claim
 1. 10. A stacked body, comprising: the cured product according to claim 9 as a recording layer; a support disposed on the upper and/or lower sides of the recording layer.
 11. A holographic recording medium obtained by a process comprising subjecting the cured product according to claim 9 to interference exposure.
 12. The holographic recording medium according to claim 11, wherein the holographic recording medium is a light guide plate for augmented reality glasses.
 13. A holographic recording medium obtained by a process comprising subjecting the stacked body according to claim 10 to interference exposure.
 14. The holographic recording medium composition according to claim 1, wherein the compound represented by formula (I) is a compound represented formula (Ib):

wherein, in formula (Ib), R^(A) is the same as that in formula (I).
 15. The holographic recording medium composition according to claim 1, wherein the compound represented by formula (I) is a compound represented formula (Ic):

wherein, in formula (Ic), R^(A) is the same as that in formula (I).
 16. A holographic recording medium composition, comprising components (a) to (e) below, wherein component (e) comprises component (e-3) below: component (a): an isocyanate group-containing compound; component (b): an isocyanate-reactive functional group-containing compound; component (c): a polymerizable monomer; component (d): a photopolymerization initiator; component (e): a compound having an isocyanate group or an isocyanate-reactive functional group and further having a nitroxyl radical group; component (e-3): a compound having an isocyanate group or an isocyanate-reactive functional group and in which the nitroxyl radical group is not sterically hindered and which is selected from (e-3-1) to (e-3-3) below: (e-3-1): a compound in which the number of alkyl groups covalently bonded to respective two atoms covalently bonded to the nitrogen atom of the nitroxyl radical group is 0 or 1; (e-3-2): a compound in which at least one of the two carbon atoms covalently bonded to the nitrogen atom of the nitroxyl radical group is covalently bonded to at least one hydrogen or halogen atom; and (e-3-3): a compound in which both of the two carbon atoms covalently bonded to the nitrogen atom of the nitroxyl radical group are each covalently bonded to at least one hydrogen or halogen atom, wherein a molar ratio of component (e-3) to component (d) (component (e-3)/component (d)) in the composition is 1.43 or more and 10 or less.
 17. A holographic recording medium composition, comprising components (a) to (e) below, wherein component (e) comprises component (e-4) below: component (a): an isocyanate group-containing compound; component (b): an isocyanate-reactive functional group-containing compound; component (c): a polymerizable monomer; component (d): a photopolymerization initiator; component (e): a compound having an isocyanate group or an isocyanate-reactive functional group and further having a nitroxyl radical group; and component (e-4): a compound having an isocyanate group or an isocyanate-reactive functional group and including a nitroxyl radical group-containing mother compound substituted with the isocyanate group or the isocyanate-reactive functional group, the nitroxyl radical group-containing mother compound having a redox potential of 280 mV or less, wherein a molar ratio of component (e-4) to component (d) (component (e-4)/component (d)) in the composition is 1.43 or more and 10 or less. 